What came first: the chicken or the egg? Or this ancient philosophical question framed theologically: What came first: Life from the dead or faith?
One of the passages that answers this question is found in 2Cor 3 in which Paul is discussing the superiority of New Covenant over the Old.
2 Cor 3:7-16
7 Now if the ministry that brought death, which was engraved in letters on stone, came with glory, so that the Israelites could not look steadily at the face of Moses because of its glory, fading though it was, 8 will not the ministry of the Spirit be even more glorious? 9 If the ministry that condemns men is glorious, how much more glorious is the ministry that brings righteousness! 10 For what was glorious has no glory now in comparison with the surpassing glory. 11 And if what was fading away came with glory, how much greater is the glory of that which lasts!
12 Therefore, since we have such a hope, we are very bold. 13 We are not like Moses, who would put a veil over his face to keep the Israelites from gazing at it while the radiance was fading away. 14 But their minds were made dull, for to this day the same veil remains when the old covenant is read. It has not been removed, because only in Christ is it taken away. 15 Even to this day when Moses is read, a veil covers their hearts. 16 But whenever anyone turns to the Lord, the veil is taken away.
NIV
Another hermeneutical principle that is extremely important to adopt and apply with many passages is the order of words or phrases as they appear in any given passage.
So Paul here is teaching that to this very day (obviously in Paul's time) the Jews in particular were not able to comprehend the Gospel of the New Covenant because their minds were made dull. And this lack of understanding, Paul expressed in terms of the metaphor "veil", since he was drawing an analogy between the veil in the OC under Moses and the same veil that remains in this NC economy. We can know that Paul had primarily Jews in mind since he refers to the "when the old covenant is read".
This "veil" persisted in Paul's day because "only in Christ is it taken away"! It is not taken away outside of Christ. It is not taken away by the Jews first coming to faith and repentance.
But what about v.16, right? But remember now: Verse 14 precedes v. 16! This latter verse says "But WHENEVER anyone turns to the Lord, the veil is taken away". But...notice what the verse does NOT say: It does NOT say, "But AFTER anyone turns..." So what is Paul getting at here? Well first, he makes it plain that the veil is only lifted by being in Christ. That much is very clear. All verse 16 is saying is HOW anyone can know the veil has been lifted. How people can know that it has been lifted -- that God has indeed performed a work in their heart -- is by their faith and repentance (turning FROM sin TO the Lord) which is when they turned to the Lord. Whenever anyone turns to the Lord, this immediately implies repentance, i.e. turning from sin. One cannot turn to the Lord, apart from turning from evil. Their faith and repentance gives affirmation to God's antecedent supernatural work in their hearts. This is how saints become aware of God's work and power in their lives. This is how they know the veil has been supernaturally and effectually lifted.
I have another passage in mind, but it'll have to wait until later.