The natural man cannot receive and understand the things of the Spirit of God 1 Cor 2 14. The Gospel is hidden from the lost 2 Cor 4:3-4
I am going to take the time to teach you an important truth about studying the word of God. The idea is to examine every word and believe the words. Try to find out how God uses words because if he is nothing else, he is consistent. I will use 2 Cor 3-4 that you have referenced as my point.
2 Cor 4:3 But if our gospel be hid, it is hid to them that ar
e lost:
4 In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them.
First of all, Paul wrote this letter. He is the apostle to the gentiles and he is a Jew. His job is to make everyone understand what God is doing in this age when gentiles for the first time in history have been included into the redemptive plan of God because of the unbelief of Israel, to whom all his promises of salvation were given and through whom all his salvation purposes were made. Only a remnant of the Israelites believed and so God the Father gave to Jesus Christ all the Israelites who would believe in him. They were all the Father's and the Father could give them to his Son if he wanted to. And he did want to. Remember, Israel were the LOST sheep, scattered on a hundred hills and without a shepherd. All the Israelites who would come to Christ did come to him and yet his house was not full. So, what did God do? He included the gentiles who would come. The gentiles were not lost. They were never God's people or sheep or anything else. The metaphor for gentiles is "DOGS." That is the long and the short of it. In all of Paul's 13 letters to gentiles he did not one time refer to them as being "lost." They were not lost, they were unsaved. They were sinners. They were under the authority of governments who were under the ruling principles of Satan for the most part. When we get to this passage in 2 Cor 4 and find the word lost being used, it is well for us to look at those to whom he refers and keep in context.
The chapter begins with the word "therefore." This means Paul is not beginning a new thought with these words but that he is drawing a conclusion from what he has just said. He is adding information that requires we read the previous thoughts that he wrote down. So, why don't we just do that. I am going to quote those thought here;
2 Cor 3:1 Do we begin again to commend ourselves? or need we, as some others, epistles of commendation to you, or letters of commendation from you?
2 Ye are our epistle written in our hearts, known and read of all men:
3 Forasmuch as ye are manifestly declared to be the epistle of Christ ministered by us, written not with ink, but with the Spirit of the living God; not in tables of stone, but in fleshy tables of the heart.
4 And such trust have we through Christ to God-ward:
5 Not that we are sufficient of ourselves to think any thing as of ourselves; but our sufficiency is of God;
6 Who also hath made us able ministers of the new testament; not of the letter, but of the spirit: for the letter killeth, but the spirit giveth life.
7 But if the ministration of death, written and engraven in stones, was glorious, so that the children of Israel could not stedfastly behold the face of Moses for the glory of his countenance; which glory was to be done away:
Transitioning from the Old Testament to the New and drawing a contrast. The difference is the operative principle of divine dealing of OT law and NT righteousness. It i works in the old and a gift in the new.
8 How shall not the ministration of the spirit be rather glorious?
9 For if the ministration of condemnation (The OT) be glory, much more doth
the ministration of righteousness exceed in glory.
10 For even that which was made glorious had no glory in this respect, by reason of the glory that excelleth.
11 For if that which is done away (the OT) was glorious, much more that which remaineth is glorious.
12 Seeing then that we have such hope, we use great plainness of speech:
13 And not as Moses, which put a veil over his face, that the children of Israel could not stedfastly look to the end of that which is abolished

There was glory in the OT but very pale compared to the NT)
Their eyes were not blinded that they could not look upon the new, but their minds were blinded so they could not look.
14 But their minds were blinded: for until this day remaineth the same vail untaken away in the reading of the old testament; which vail is done away in Christ.
15 But even unto this day, when Moses is read, the vail is upon their heart.
16 Nevertheless when it (their heart) shall turn to the Lord, the vail shall be taken away.
17 Now the Lord is that Spirit: and where the Spirit of the Lord is, there is liberty.
18 But we all, with open face beholding as in a glass the glory of the Lord,
are changed into the same image from glory to glory, even as by the Spirit of the Lord.
The image of the Lord is trinitarian, achieved for any man who believes the gospel of Jesus Christ and achieved by the presence of the Spirit of God dwelling in him.
The next word is THEREFORE. This is the context and it lets us know that God intended for Israel to transition with him from the old to the new. but they refused. All these truths we are given in the beginning of this age are so very important so we can know why God has included gentiles and given us the wonderful privilege of being in his church. What if we did not have this information? Would we know what God is doing?
Lk 14:16 Then said he unto him, A certain man made a great supper, and bade many:
17 And sent his servant at supper time to say to them that were bidden, Come; for all things are now ready.
18 And they all with one consent began to make excuse. The first said unto him, I have bought a piece of ground, and I must needs go and see it: I pray thee have me excused.
19 And another said, I have bought five yoke of oxen, and I go to prove them: I pray thee have me excused.
20 And another said, I have married a wife, and therefore I cannot come.
21 So that servant came, and shewed his lord these things. Then the master of the house being angry said to his servant, Go out quickly into the streets and lanes of the city, and bring in hither the poor, and the maimed, and the halt, and the blind.
22 And the servant said, Lord, it is done as thou hast commanded, and yet there is room.
23 And the lord said unto the servant,
Go out into the highways and hedges, and compel them to come in, that my house may be filled.
24 For I say unto you, That none of those men which were bidden shall taste of my supper.
The Jews were bidden but the would not come so the gentiles were called to fill his house.
Rom 11:13 For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles, I magnify mine office:
18 Boast not against the branches. But if thou boast, thou bearest not the root, but the root thee.
19 Thou wilt say then,
The branches were broken off, that I might be grafted in.
20 Well;
because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear:
21 For if God spared not the natural branches, take heed lest he also spare not thee.
22 Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off.
25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel,
until the fulness of the Gentiles be come in
Until, and then what?
26 And so all Israel shall be saved:
The veil will be taken away in the next age for the Jews. I personally am glad to have this information given to me and I want to process it properly.