And David inquired of the LORD, saying, Shall I go up to the Philistines? wilt thou deliver them into mine hand? And the LORD said unto David, Go up: for I will doubtless deliver the Philistines into thine hand. 2 Sam 5:19
One of the questions commonly asked by those who deny the gospel of Christ the effectual, sovereign Redeemer is, "If God has a chosen people and Christ only died for those, then why preach the gospel?" In our text King David has been assured of certain victory by the Lord whom He absolutely trusted and whose word he considered to be as good as the very deed. Suppose that David's armor-bearer asked him this question, "King David, God has assured you victory beyond a shadow of a doubt in this pending battle with the Philistines, so why bother going to fight them since you already know you're going to win?" I believe David's answer would be the same as my answer to the sentimental, free-will worshippers of the weak, trying-to-but-can't god of today's religion.
First of all, because God told me to. God did not say, "I will doubtless deliver the Philistines into thine hand, so there won't be any need for you to go up." What God did say was, "GO UP!" Just so, He has instructed His preachers to go and preach the gospel to every creature (Mark 16:15). The fact that when that gospel is preached, His word will accomplish the purpose for which it was sent without possibility of failure (Is. 55:11) is all the more reason to preach it! The God who ordained the end result of that preaching is the same God who ordained the means or preaching of it (I Cot. 1:21).
Secondly, because there is a great need. God would not have said to go otherwise. There is an enemy that needs a whoopin' and there is a harvest of God that needs reapin'.
Thirdly, because it is my absolute pleasure to do so. Don't you know when David asked God if he should go up and God personally said to David, "Go up" it must have caused a gleam in David's eye. What an anticipation and joy must have filled him at the very prospect of fighting on behalf of his God with an infallible assurance of ultimate victory. We who preach the gospel of the sovereign grace of the almighty God do so with a similar gleam in our eye.
Fourthly, for His great glory. Every flash of David's sword signaled the greatness of David's God, and every clank of steel sounded out His awesome praise. When God's man opens his mouth, out flows the glory and praise of Him who washes sinners clean in His own precious blood. What grace that God has ordained it so!
When you think about it, it's kind of a silly question, isn't it?