There is a great difference in preaching about Christ and preaching Christ. Many men proclaim truths about the Lord Jesus, but fail to proclaim Him.
Salvation is the personal knowledge of that One who came to seek and save that which was lost.
I may know things that have been accomplished by the President of the United States, but not know him at all. Preaching about Christ may reveal a lot of historical facts that any carnal heart can understand, but that type preaching is not the preaching that is blessed to the salvation of sinners. The power of God unto salvation is the preaching of the Gospel, and the Gospel is always concerning Christ and Him crucified. Many men know much about one sent from heaven, but that in itself is not enough. I must be known and loved by Him effectually.