The expression “whole counsel of God” refers to God’s counsels of eternity, in which He planned all things that will come to be. God knows the end from the beginning, and He knows what He intends to accomplish, which He has revealed in the Scriptures.
In Acts 20, Paul assured the Ephesian Christians that he had preached to them the whole counsel of God. Now, that does not mean the whole mind of God—the infinite mind of God cannot be preached by a finite preacher. In that passage, however, I think Paul means, “I told you all the important things that God has revealed that you must know for your salvation and service to Him.”
Paul, particularly in speaking to the Ephesian elders, reminds us that we need to know the fullness of the revelation that is crucial for our lives and our knowledge of God’s truth, and we need preachers who will bring that to us. That is why we need preachers who don’t just ride hobbies or preach in a scattershot way, but instead help us grasp the fullness of what God has revealed. That often means preaching in an expository way through the books of the Bible.
Contrary to some of my Presbyterian friends, I believe it is also useful to preach through a catechism, such as the Heidelberg Catechism, that takes us from topic to topic in the fullness of what God has revealed and reminds the congregation of those things. You can preach many books of the Bible but never get to the Lord’s Supper, for example, but the Lord’s Supper is very important for a Christian. So, ministers need to figure out strategies for bringing the whole counsel of God to bear in their preaching and teaching.