Romans 1:20

For since the creation of the world his invisible attributes—his eternal power and divine nature—have been clearly seen, because they are understood through what has been made. So people are without excuse.
Romans 1:20, New English Translation (NET)
 
No excuse. There isn't anyone who stands before God in judgment who will be able to claim they didn't know. There are many of us who have found this difficult to understand, let alone explain to others, myself included. However, just because we may struggle with it doesn't mean it's validity is in question. There are many, including Christians, who have this silly idea that if there's something in Scripture we don't understand, or that we don't think is right, it is therefore not true. Really? We are God's judges? I know that we humans think very highly of ourselves, but that is probably going a bit far.
 
Albert Barnes explains it like this in his Notes on the Bible:
 
For the invisible things of him - The expression “his invisible things” refers to those things which cannot be perceived by the senses. It does not imply that there are any things pertaining to the divine character which may be seen by the eye; but that there are things which may be known of him, though not discoverable by the eye. We judge of the objects around us by the senses, the sight, the touch, the ear, etc. Paul affirms, that though we cannot judge thus of God, yet there is a way by which we may come to the knowledge of him. What he means by the invisible things of God he specifies at the close of the verse, “his eternal power and Godhead.” The affirmation extends only to that; and the argument implies that that was enough to leave them without any excuse for their sins.
 
From the creation of the world - The word “creation” may either mean the “act” of creating, or more commonly it means “the thing created,” the world, the universe. In this sense it is commonly used in the New Testament; compare Mrk_10:6; Mrk_13:19; Mrk_16:5; Rom_1:25; 2Co_5:17; Gal_6:15; Col_1:15, Col_1:23; Heb_4:13; Heb_9:11; 1Pe_2:13; 2Pe_3:4; Rev_3:14. The word “from” may mean “since,” or it may denote “by means of.” And the expression here may denote that, as an historical fact, God “has been” “known” since the act of creation; or it may denote that he is known “by means of” the material universe which he has formed. The latter is doubtless the true meaning. For,
 
(1) This is the common meaning of the word “creation;” and,
 
(2) This accords with the design of the argument.
 
It is not to state an historical fact, but to show that they had the means of knowing their duty within their reach, and were without excuse. Those means were in the wisdom, power, and glory of the universe, by which they were surrounded.
 
Are clearly seen - Are made manifest; or may be perceived. The word used here does not occur elsewhere in the New Testament.
 
Being understood - His perfections may be investigated, and comprehended by means of his works. They are the evidences submitted to our intellects, by which we may arrive at the true knowledge of God.
 
Things that are made - By his works; compare Heb_11:3. This means, not by the original “act” of creation, but by the continual operations of God in his Providence, by his doings, ποιήμασιν poiēmasin, by what he is continually producing and accomplishing in the displays of his power and goodness in the heavens and the earth. What they were capable of understanding, he immediately adds, and shows that he did not intend to affirm that everything could be known of God by his works; but so much as to free them from excuse for their sins.
 
His eternal power - Here are two things implied.
 
(1) That the universe contains an exhibition of his power, or a display of that attribute which we call “omnipotence;” and,
 
(2) That this power has existed from eternity, and of course implies an eternal existence in God.
It does not mean that this power has been exerted or put forth from eternity, for the very idea of creation supposes that it had not, but that there is proof, in the works of creation, of power which must have existed from eternity, or have belonged to an eternal being. The proof of this was clear, even to the pagan, with their imperfect views of creation and of astronomy; compare Psa_19:1-14. The majesty and grandeur of the heavens would strike their eye, and be full demonstration that they were the work of an infinitely great and glorious God. But to us, under the full blaze of modern science, with our knowledge of the magnitude, and distances, and revolutions of the heavenly bodies, the proof of this power is much more grand and impressive. We may apply the remark of the apostle to the present state of the science, and his language will cover all the ground, and the proof to human view is continually rising of the amazing power of God, by every new discovery in science, and especially in astronomy. Those who wish to see this object presented in a most impressive view, may find it done in Chalmer’s Astronomical Discourses, and in Dick’s Christian Philosopher. Equally clear is the proof that this power must have been eternal. If it had not always existed, it could in no way have been produced. But it is not to be supposed that it was always exerted, any more than it is that God now puts forth all the power that he can, or than that we constantly put forth all the power which we possess. God’s power was called forth at the creation. He showed his omnipotence; and gave, by that one great act, eternal demonstration that he was almighty; and we may survey the proof of that, as clearly as if we had seen the operation of his hand there. The proof is not weakened because we do not see the process of creation constantly going on. It is rather augmented by the fact that he sustains all things, and controls continually the vast masses of matter in the material worlds.
 
Godhead - His deity; divinity; divine nature, or essence. The word is not used elsewhere in the New Testament. Its meaning cannot therefore be fixed by any parallel passages. It proves the truth that the supremacy, or supreme divinity of God, was exhibited in the works of creation, or that he was exalted above all creatures and things. It would not be proper, however, to press this word as implying that all that we know of God by revelation was known to the pagan; but that so much was known as to show his supremacy; his right to their homage; and of course the folly and wickedness of idolatry. This is all that the argument of the apostle demands, and, of course, on this principle the expression is to be interpreted.
 
So that they are without excuse - God has given them so clear evidence of his existence and claims, that they have no excuse for their idolatry, and for hindering the truth by their iniquity. It is implied here that in order that people should be responsible, they should have the means of knowledge; and that he does not judge them when their ignorance is involuntary, and the means of knowing the truth have not been communicated. But where people have these means within their reach, and will not avail themselves of them, all excuse is taken away. This was the case with the Gentile world. They had the means of knowing so much of God, as to show the folly of worshipping dumb idols; compare Isa_44:8-10. They had also traditions respecting his perfections; and they could not plead for their crimes and folly that they had no means of knowing him. If this was true of the pagan world then, how much more is it true of the world now?
 
And especially how true and fearful is this, respecting that great multitude in Christian lands who have the Bible, and who never read it; who are within the reach of the sanctuary, and never enter it; who are admonished by friends, and by the providences of God, and who regard it not; and who look upon the heavens, and even yet see no proof of the eternal power and Godhead of him who made them all! Nay, there are those who are apprized of the discoveries of modern astronomy, and who yet do not seem to reflect that all these glories are proof of the existence of an eternal God; and who live in ignorance of religion as really as the pagan, and in crimes as decided and malignant as disgraced the darkest ages of the world. For such there is no excuse, or shadow of excuse, to be offered in the day of doom. And there is no fact more melancholy in our history, and no one thing that more proves the stupidity of people, than this sad forgetfulness of Him that made the heavens, even amid all the wonders and glories that have come fresh from the hand of God, and that everywhere speak his praise.