James 1:6

But let him ask in faith, without wavering. For he who wavers is like a wave of the sea, driven and tossed with the wind.
James 1:6 Modern English Version (MEV)
 
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But let him ask in faith - See the passages referred to in Jas_1:5. Compare the Mat_7:7 note, and Heb_11:6 note. We cannot hope to obtain any favor from God if there is not faith; and where, as in regard to the wisdom necessary to guide us, we are sure that it is in accordance with his will to grant it to us, we may come to him with the utmost confidence, the most entire assurance, that it will be granted. In this case, we should come to God without a doubt that, if we ask with a proper spirit, the very thing that we ask will be bestowed on us. We cannot in all other cases be so sure that what we ask will be for our good, or that it will be in accordance with his will to bestow it; and hence, we cannot in such cases come with the same kind of faith. We can then only come with unwavering confidence in God, that he will do what is right and best; and that if he sees that what we ask will be for our good, he will bestow it upon us. Here, however, nothing prevents our coming with the assurance that the very thing which we ask will be conferred on us.
 
Nothing wavering - (μηδὲν διακρινόμενος mēden diakrinomenos.) “Doubting or hesitating as to nothing, or in no respect.” See Act_20:20; Act_11:12. In regard to the matter under consideration, there is to be no hesitancy, no doubting, no vacillation of the mind. We are to come to God with the utmost confidence and assurance.
 
For he that wavereth, is like a wave of the sea ... - The propriety and beauty of this comparison will be seen at once. The wave of the sea has no stability. It is at the mercy of every wind, and seems to be driven and tossed every way. So he that comes to God with unsettled convictions and hopes, is liable to be driven about by every new feeling that may spring up in the mind. At one moment, hope and faith impel him to come to God; then the mind is at once filled with uncertainty and doubt, and the soul is agitated and restless as the ocean. Compare Isa_57:20. Hope on the one hand, and the fear of not obtaining the favor which is desired on the other, keep the mind restless and discomposed.
 
Albert Barnes’ Notes on the Bible