Do you ever feel like losing heart when you try to tell people about Jesus and it just doesn't seem to penetrate at all? Paul felt the same as you. His answer in 2 Cor 4 is that the New Covenant is mighty and does win. God will out. Even if we aren't seeing the results we would like at the moment, it shouldn't shake our confidence, because our confidence is in the Lord not in our visible results. Paul says:

We don't lose heart because we have this ministry through the mercy of God v1
Therefore, unlike some who are desperate for visible successes, we don't commend ourselves and our message by distorting God's Word and deceiving people v2
But we do commend ourselves to people's consciences by plainly setting forth the truth about God v3
If they don't receive it, it is because Satan has blinded their minds v4
We are confident, nevertheless, because we aren't preaching ourselves but Jesus as Lord. We serve people inorder that they get to know him v5
We are confident because the power doesn't come from us but from God. Results are in his hand so he gets the glory v7
Therefore we can be extremely hard-pressed but not despairing, crushed, abandoned or destroyed v8-9
In fact sharing in Jesus sufferings (ie suffering to extend his offer of reconcilliation with God) means we also share in his life and delight in the new life he is giving to others v10-12
We are confident to speak in the face even of death because we know that God who raised Jesus will raise us with him v13-14
By this means grace is reaching more and more people, regardless of the hindrances. Satan may be blinding minds, but God is opening many more eyes v15
Therefore DON'T LOSE HEART, because current momentary troubles are far outweighed by the eternal glory v16
Fix your eyes not on the temporary troubles but on the eternal glory v17

(There is an obvious question from this chapter that I don't quite think I have an answer that I yet find quite satisfying (ie I have some answers but not ones that exhaust all the questions this throws up for me): why does God allow Satan to blind the minds of unbelievers? Praying for more light on this one from the word. In situations like this it is best to just go with what it says and admit to not knowing until more understanding comes)