HAT is it that makes some gospel preaching fruitless? Not “what makes preaching in general fruitless?” Preaching without the gospel is senseless, & we should expect it to be fruitless. But assuming that the gospel is being preached, any failure is in the listener. Verse 2 does not say that the preaching of the gospel is unprofitable or ineffective in some instances because (for example) the preaching style is. When the true gospel is preached yet no one responds, the problem is a lack of faith in the listeners (vs. 6). Truth without faith brings deeper guilt (or worse, spiritual cynicism). Attempts at faith without truth is only misguided foolishness. But together, humble faith & the gospel changes lives.
Now, if a man has no faith, what does he have? The answer: his deeds. And, THAT is a problem (Isaiah 64:6). Our deeds (good or bad by our own estimation) do not lift us up to God. Our works can never add one nanometer to our spiritual elevation. Righteousness is imputed. To those who hear & heed, we rest in God's promises & in His goodness (rather than depending our own puny efforts – vs. 10). This is the aim of the gospel. The gospel is the good news of God's salvation (by grace through faith in the finished work of Christ). But again, if the audience has no faith, then the good news only increases our accountability; our culpability. Without faith accompanying the gospel, the blame on us is only intensified.
Now, what if a man's faith is lacking, waning, or absent altogether? Hebrews 4:11 says, "Labor to enter into that rest." Coming to Christ in genuine faith is the most urgent endeavor a person can undertake. Obviously. the author of Hebrews is not promoting a works salvation. But he is pointing to the necessity of sincerity.