The question of God's existence and the meaning of life are not necessarily connected. A Deist believes that God created the universe, then took no further interest in it, so the universe in that case could have no meaning at all. An atheist may not believe in God, but could believe that we apply our own meaning to the universe. For example, that the "purpose" of the universe is to develop intelligent life so it can observe itself.
By the way, Lewis was an outstanding writer, but a terrible philosopher. He should have stuck to fiction; his "apologetics" are just an embarrassment and only detract from his legacy.