Q.3: How may we know there is a God?
A. The light of nature in man and the works of God plainly declare there is a God; but His Word and Spirit only do it fully and effectually for the salvation of sinners.
Because what may be known of God is manifest in them, for God has shown it to them. For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse.
Romans 1:19-20
Daily Baptist Confession 