Answer to a Correspondent.

What is it that gives capacity to the believer? Is it new birth, or is it the possession of the Holy Spirit? — Gateshead-On-Tyne.
YOUR question is a trifle difficult to answer inasmuch as the word capacity may mean somewhat different things to different minds.
In Romans 7 we find the struggles of a new-born soul. We can see that a capacity exists for recognizing and approving what is good, though the power to execute it is lacking. There does not appear to be as yet a capacity for understanding, though there are divinely-implanted instincts.
In 1 John 2:20-27,20But ye have an unction from the Holy One, and ye know all things. 21I have not written unto you because ye know not the truth, but because ye know it, and that no lie is of the truth. 22Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son. 23Whosoever denieth the Son, the same hath not the Father: (but) he that acknowledgeth the Son hath the Father also. 24Let that therefore abide in you, which ye have heard from the beginning. If that which ye have heard from the beginning shall remain in you, ye also shall continue in the Son, and in the Father. 25And this is the promise that he hath promised us, even eternal life. 26These things have I written unto you concerning them that seduce you. 27But the anointing which ye have received of him abideth in you, and ye need not that any man teach you: but as the same anointing teacheth you of all things, and is truth, and is no lie, and even as it hath taught you, ye shall abide in him. (1 John 2:20‑27) we read of the “unction” or “anointing,” —referring to the Spirit; by it we “know.” The same anointing “teacheth you of all things.” Capacity for the understanding of the things of God, the power by which we consciously possess them, is by the Spirit This is illustrated in the history of the disciples. Born again as they were, they had divinely-given instincts, and hence they recognized in Jesus the Christ of God and clung to Him. Yet they did not understand in any proper way the wonderful teaching that He gave them until He was gone and the Spirit was given. Again and again He had to comment on their lack of understanding, for “These things understood not His disciples at the first: but when Jesus was glorified, then remembered they that these things were written of Him” (John 12:1616These things understood not his disciples at the first: but when Jesus was glorified, then remembered they that these things were written of him, and that they had done these things unto him. (John 12:16)). Speaking of this the Lord said, “At that day ye shall know...” (John 14:2020At that day ye shall know that I am in my Father, and ye in me, and I in you. (John 14:20)).
In short, therefore, we should answer the question you raise by saying that, while as a result of new birth we possess what we may call an instinctive capacity — a capacity of divinely given desires and feelings, our capacity for an intelligent reception of the things of God lies in our having received the Holy Spirit — and in our walking so as not to grieve Him.