One way to look at why preaching is considered foolish, is through the understanding that all is of God. If all will be saved, what's the point in preaching? Maybe, the preaching is intended only for His Elect at this time.
Here's a great quote from Ray on the subject that might help.
https://bible-truths.com/forums/index.php/topic,7500.0.htmlWell I'll tell you one thing for sure: you are at least thinking. Your "P.S." was a good question. If God knows all, and has predetermined all, and "operates ALL according to the counsel of His own will," then why should we "do" or "not do" ANYTHING? I think that is the gist of your question.
There are several reasons for this. I think the main reason is that although we know (or at least those to whom God has opened up this understanding) that "God is operating ALL" for the benefit of His creatures, we still don't know always, when, where, or how He is going to do this. We don't know what a day will bring. We MUST rely on God COMPLETELY for all our salvation's. Be it safety in traffic, financial supply, health needs, social intercourse's, etc., etc., etc. We never know from hour to hour or from day to day just what will transpire in our lives. This should DRIVE us to our Heavenly Father. And it does. It works. God knows what he is doing.
People ask me WHY do I do what I do? Why spend all the time and trouble and expense if God is going to save EVERYONE ANYWAY? I'll tell you why. Because God PUTS IT IN OUR HEARTS to do so. The Scriptures say that God could raise up stones to do His work. But He doesn't. He uses "the foolishness of preaching" to get His word out.
Maybe an analogy will help. Suppose you had a bad disease and there was only one doctor and one source of medication that could save you from your deadly disease. Even if this doctor agreed to come to your aid, you would STILL do everything in your power to ingratiate yourself with this man, wouldn't you? Wouldn't you be kind and respectful to him? Wouldn't you even show a bit of reverence for this man, the only man, who can save you from your disease? Sure you would. And so it is with God. He is our Saviour and our hope and expectation in EVERYTHING! The Scriptures say that "in God we live and move and breathe and ARE"!
Now I will pass on another bit of wisdom. The people to whom God has opened up this understanding are delighted that this is the way things really are. They would have it no other way, than that it is GOD and God alone Who determines our every footstep. However, those who feel this is NOT RIGHT, or they do NOT APPROVE of God operating ALL (EVERYTHING), or feel that man is far superior in his "will" than the Scriptures allow, this people DON'T BELIEVE GOD IS OPERATING ALL IN THE FIRST PLACE! So they can't find too much fault with this truth, because they don't believe it anyhow.