Romans 1:18-19

For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of people who suppress the truth by their unrighteousness, because what can be known about God is plain to them, because God has made it plain to them.
Romans 1:18-19, New English Translation (NET)
 
Not only is ignorance no excuse, but ignorance can't even be claimed. Sin is more than simply missing the mark; it is actively suppressing the truth about God and from God. Sin is revolt against God, active and defiant. The ungodly, the unrighteous, sinners, cannot claim they don't know.
 
A quick word about "ungodliness and unrighteousness." Ungodliness can generally be understood as without God, not having God as their center or source. Unrighteousness can generally be understood as being similar to injustice or without justice in what they say and do, particularly toward others. Keeping this in mind will help us to better understand the next few verses.
 
Because we are approaching verses that are often a bit confusing and misunderstood, we will need to look to Albert Barnes’ Notes on the Bible to give us some additional details:
 
For - This word denotes that the apostle is about to give a reason for what he had just said. This verse commences the argument of the Epistle. an argument designed to establish the proposition advanced in Rom_1:17. The proposition is, that God’s plan of justification is revealed in the gospel. To show this, it was necessary to show that all other plans had failed; and that there was need of some new plan or scheme to save people. To this he devotes this and the two following chapters. The design of this argument is, to show that people were sinners. And in order to make this out, it was necessary to show that they were under law. This was clear in regard to the Jews. They had the Scriptures; and the apostle in this chapter shows that it was equally clear in regard to the Gentiles, and then proceeds to show that both had failed of obeying the Law. To see this clearly it is necessary to add only, that there can be but two ways of justification conceived of; one by obedience to law, and the other by grace. The former was the one by which Jews and Gentiles had sought to be justified; and if it could be shown that in this they had failed, the way was clear to show that there was need of some other plan.
 
The wrath of God - ὀργὴ Θεοῦ orgē Theou. The word rendered “wrath” properly denotes that earnest appetite or desire by which we seek anything, or an intense effort to obtain it. And it is particularly applied to the desire which a man has to take vengeance who is injured, and who is enraged. It is thus synonymous with revenge. Eph_4:31, “let all bitterness, and wrath, etc.; Col_3:8, “anger, wrath, malice,” etc.; 1Ti_2:8; Jas_1:19. But it is also often applied to God; and it is clear that when we think of the word as applicable to him, it must be divested of everything like human passion, and especially of the passion of revenge. As he cannot be injured by the sins of people Job_25:6, he has no motive for vengeance properly so called, and it is one of the most obvious rules of interpretation that we are not to apply to God passions and feelings which, among us, have their origin in evil.
 
In making a revelation, it was indispensable to use words which people used; but it does not follow that when applied to God they mean precisely what they do when applied to man. When the Saviour is said Mrk_3:5 to have looked on his disciples with anger (Greek, “wrath,” the same word is here), it is not to be supposed that he had the feelings of an implacable man seeking vengeance. The nature of the feeling is to be judged of by the character of the person. So, in this place, the word denotes the “divine displeasure” or “indignation” against sin; the divine purpose to “inflict punishment. It is the opposition of the divine character against sin;” and the determination of the divine mind to express that opposition in a proper way, by excluding the offender from the favors which he bestows on the righteous. It is not an unamiable, or arbitrary principle of conduct. We all admire the character of a father who is opposed to disorder, and vice, and disobedience in his family, and who expresses his opposition in a proper way.
We admire the character of a ruler who is opposed to all crime in the community, and who expresses those feelings in the laws. And the more he is opposed to vice and crime, the more we admire his character and his laws; and why shall we be not equally pleased with God, who is opposed to all crime in all parts of the universe, and who determines to express it in the proper way for the sake of preserving order and promoting peace? The phrase “divine displeasure” or “indignation,” therefore, expresses the meaning of this phrase; see Mat_3:7; Luk_3:7; Luk_21:23; Jhn_3:36; Rom_2:5, Rom_2:8; Rom_3:5; Rom_4:15; Rom_5:9; Rom_9:22; Rom_12:19; Rom_13:4-5; Eph_2:3; Eph_5:6; 1Th_1:10; 1Th_2:16, etc. The word occurs 35 times in the New Testament.
 
Is revealed - That is, revealed to the Jews by their Law; and to the Gentiles in their reason, and conscience, as the apostle proceeds to show.
 
From heaven - This expression I take to mean simply that the divine displeasure against sin is made known by a divine appointment; by an arrangement of events, communications, and arguments, which evince that they have had their origin in heaven; or are divine. How this is, Paul proceeds to state, in the works of creation, and in the Law which the Hebrews had. A variety of meanings have been given to this expression, but this seems the most satisfactory. It does not mean that the wrath will be sent from heaven; or that the heavens declare his wrath; or that the heavenly bodies are proofs of his wrath against sin; or that Christ, the executioner of wrath, will be manifest from heaven (Origen, Cyril, Beza, etc.); or that it is from God who is in heaven; but that it is by an arrangement which shows that it had its origin in heaven. or has proofs that it is divine.
 
Against all ungodliness - This word properly means “impiety” toward God, or neglect of the worship and honor due to him. ἀσέβειαν asebeian. It refers to the fact that people had failed to honor the true God, and had paid to idols the homage which was due to him. Multitudes also in every age refuse to honor him, and neglect his worship, though they are not idolaters. Many people suppose that if they do not neglect their duty to their fellow-men, if they are honest and upright in their dealings, they are not guilty, even though they are not righteous, or do not do their duty to God; as though it were a less crime to dishonor God than man; and as though it were innocence to neglect and disobey our Maker and Redeemer. The apostle here shows that the wrath of God is as really revealed against the neglect of God as it is against positive iniquity; and that this is an offence of so much consequence as to be placed “first,” and as deserving the divine indignation more than the neglect of our duties toward people; compare Rom_11:26; 2Ti_2:16; Tit_2:12; Jud_1:15, Jud_1:18. The word does not occur elsewhere in the New Testament.
 
Unrighteousness of men - Unrighteousness, or iniquity toward people. All offences against our neighbor, our parents. our country, etc. The word “ungodliness” includes all crimes against God; this, all crimes against our fellow-men. The two words express what comprehends the violation of all the commands of God; “Thou shalt love the Lord thy God, etc. and thy neighbor as thyself,” Mat_22:37-40. The wrath of God is thus revealed against all human wickedness.
 
Who hold the truth - Who “keep back,” or “restrain” the truth. The word translated “hold” here, sometimes means to “maintain,” to “keep,” to “observe” 1Co_7:30; 2Co_6:12; but it also means to “hold back, to detain, to hinder.” Luk_4:42, “the people sought him (Jesus), and came to him, and stayed him.” (Greek, the same as here.) Phm_1:13, “whom I would have “retained” with me,” etc.; 2Th_2:6, “and now ye know what “withholdeth,” etc. In this place it means also that they held back, or restrained the truth, by their wickedness.
 
The truth - The truth of God, in whatever way made known, and particularly, as the apostle goes on to say, what is made known by the light of nature. The truth pertaining to his perfections, his Law, etc.
 
They hold it back. or restrain its influence.
In unrighteousness - Or rather, by their iniquity. Their wickedness is the cause why the truth had had so little progress among them, and had exerted so little influence. This was done by their yielding to corrupt passions and propensities, and by their being therefore unwilling to retain the knowledge of a pure and holy God, who is opposed to such deeds, and who will punish them. As they were determined to practice iniquity, they chose to exclude the knowledge of a pure God, and to worship impure idols, by which they might give a sanction to their lusts. Their vice and tendency to iniquity was, therefore, the reason why they had so little knowledge of a holy God; and by the love of this, they held back the truth from making progress, and becoming diffused among them.
 
The same thing is substantially true now. People hold back or resist the truth of the gospel by their sins in the following ways.
 
(1) People of influence and wealth employ both, in directly opposing the gospel.
 
(2) People directly resist the doctrines of religion. since they know they could not hold to those doctrines without abandoning their sins.
 
(3) People who resolve to live in sin, of course, resist the gospel, and endeavor to prevent its influence.
 
(4) Pride, and vanity, and the love of the world also resist the gospel, and oppose its advances.
 
(5) Unlawful business - business that begins in evil, and progresses, and ends in evil - has this tendency to hold back the gospel. Such is the effect of the traffic in ardent spirits, in the slave-trade, etc. They begin in the love of money, the root of all evil 1Ti_6:10; they progress in the tears and sorrows of the widow, the orphan, the wife, the sister, or the child; and they end in the deep damnation of multitudes in the world to come. Perhaps there has been nothing that has so much held back the influence of truth, and of the gospel, as indulgence in the vice of intemperance, and traffic in liquid fire.
 
(6) Indulgence in vice, or wickedness of any kind, holds back the truth of God. People who are resolved to indulge their passions will not yield themselves to this truth. And hence, all the wicked, the proud, and vain, and worldly are responsible, not only for their own sins directly, but for hindering, by their example and their crimes, the effect of religion on others. They are answerable for standing in the way of God and his truth; and for opposing him in the benevolent design of doing good to all people. There is nothing that prevents the universal spread and influence of truth but sin. And people of wickedness are answerable for all the ignorance and wo which are spread over the community, and which have extended themselves over the world.
 
Because - The apostle proceeds to show how it was that the pagan hindered the truth by their iniquity. This he does by showing that the truth might be known by the works of creation; and that nothing but their iniquity prevented it.
 
That which may be known of God - That which is “knowable” concerning God. The expression implies that there may be many things concerning God which cannot be known. But there are also many things which may be ascertained. Such are his existence, and many of his attributes, his power, and wisdom, and justice, etc. The object of the apostle was not to say that every thing pertaining to God could be known by them, or that they could have as clear a view of him as if they had possessed a revelation. We must interpret the expression according to the object which he had in view. That was to show that so much might be known of God as to prove that they had no excuse for their crimes; or that God would be just in punishing them for their deeds. For this, it was needful only that his existence and his justice, or his determination to punish sin, should be known; and this, the apostle affirms, was known among them, and had been from the creation of the world. This expression. therefore, is not to be pressed as implying that they knew all that could be known about God, or that they knew as much as they who had a revelation; but that they knew enough to prove that they had no excuse for their sins.
Is manifest - Is known; is understood.
 
In them - “Among” them. So the preposition “in” is often used. It means that they had this knowledge; or it had been communicated to them. The great mass of the pagan world was indeed ignorant of the true God; but their leaders, or their philosophers, had this knowledge; see the note at Rom_1:21. But this was not true of the mass, or body of the people. Still it was true that this knowledge was in the possession of man, or was “among” the pagan world. and would have spread, had it not been for the love of sin.
 
God hath showed it to them - Compare Jhn_1:9. He had endowed them with reason and conscience Rom_2:14-15; he had made them capable of seeing and investigating his works; he had spread before them the proofs of his wisdom, and goodness, and power, and had thus given them the means of learning his perfections and will.