In reality, with all of his problems, it is actually doubtful that Tim Keller actually questions if there is a God. In fact, I would argue from the Scriptures that nobody actually questions the existence of God deep down — even atheists. People will simply suppress the truth about God in unrighteousness according to Romans 1:18.
Not only does Scripture reveal that there is a God, but He has also revealed himself through His creation. One does not need to know the Scriptures to know that there is a God.
The heavens declare the glory of God, and the sky above proclaims his handiwork.Psalm 19:1
Tim Keller knows this, which makes this statement is irresponsible.
It is irresponsible because it serves to further suppress the truth about God by validating the question. Tim Keller is Reformed — at least in name — yet he does not act like a Reformed Christian. The Keller would insult the revealed God by placing a conditional upon His existence, why stop there? If His existence isn’t even necessary, then why is the fact that he would know what’s best for us necessary?
The problem here isn’t that Keller actually questions the existence of God — or even the kind of God. There is no question of “if God,” there is only the statement, “God is.” The problem here is that, in typical Tim Keller fashion, he calls in the veracity of Scripture as he subjects them to conditionals which are subject to interpretation through human reasoning.