How Did We Know That There's God?

Question: If God is Spirit, then the Spirit has no flesh and bones, therefore God is invisible. But how did we know that there is God?

Answer: We are sure that there is really God because everything that we need to know about God has been revealed to us.

19 because what may be known of God is manifest in them, for God has shown it to them.

Romans 1:19, NKJV

19 because that which is known about God is evident within them; for God made it evident to them.

Romans 1:19, New American Standard Bible