It is no wonder these truths are suppressed. For non-Christians, there is no good news at all yet. After the law comes the gospel. The message of the cross and resurrection reconciles sinners to God, and part of this reconciliation is the dispelling of intellectual darkness. The futility of thinking is gone because the hardness of heart which produced it is gone.
Hard hearts makes for soft heads.
Because the Spirit of God has taken away the heart of stone, the way the man thinks is altered forever. Everything is not done at once, but the process has begun. Good reasons, good defenses come from good hearts. The one to whom he speaks has not sanctified the Lord in his heart, and does not have a good conscience. This is why he is in intellectual darkness.