Why can't we make people understand spiritual truth by our own speaking?
Because human speaking alone cannot open the heart or spiritual eyes. Even Paul's powerful preaching could not open Lydia's heart; the Lord had to open it first, and then Paul's words took root. Without the Holy Spirit's work, our words accomplish nothing and are only a tinkling cymbal.
Les explains that in the account of Lydia, she worshipped God and heard Paul, but Scripture says her heart was opened by the Lord: "whose heart the Lord opened," 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 by his preaching. It took the work of the Holy Spirit to open Lydia's heart, and then Paul's words took root. It is no different today: you and I can do nothing, and we can never get across these truths unless the Holy Spirit opens the heart and spiritual eyes of those to whom we are speaking. Les says his words are nothing more than a tinkling cymbal unless God Himself opens hearts. Many only catch it in God's own time, not before. So proclamation requires God to be in it; He has to send us, and beautiful are the feet of those who proclaim the gospel of peace and bring glad tidings of good things.