Why can't we convince people of spiritual truths just by speaking to them?
Human speaking alone cannot convince anyone of spiritual truths, because the heart must first be opened by the Holy Spirit. Even Paul's preaching could not open Lydia's heart; the Lord opened it, and then Paul's words took root. Without that divine work, our words accomplish nothing.
Les explains that in the account of Lydia, she worshipped God and heard Paul, but Scripture says the Lord opened her heart so that she attended to the things spoken by Paul. Les emphasizes that even the great apostle Paul, with the power he had, could not open the hearts of those people with his preaching. It took the work of the Holy Spirit to open Lydia's heart, and then Paul's words took root. Les says it is no different today: you and I can do nothing and can never get across these truths unless the Holy Spirit opens the heart and spiritual eyes of those to whom we are speaking. He compares his own words to a tinkling cymbal unless God Himself opens hearts. He also notes that many ask why they did not catch this years earlier, and the answer is that it was not God's time; but in His own time, He opens hearts and spiritual understanding.