The billboards work for the same reason. Their short, simple statements are easy enough to communicate with the average commuter. When we advertise for God, the way we say what we say is important. But what we say is even more important. We do not use deception, nor do we distort the word of God.
Christians have nothing to hide.
Therefore, we should be completely candid about what we believe. Whatever we say about God and his salvation must be absolutely true. Our God is the truth. If what we say about him is anything but the truth, then we misrepresent him. We should never deceive people about our true intentions.