This is how Moses confirmed his authority in Egypt before Pharaoh: the sign of the staff turning into a snake, and the hand being turned leprous—pointing to the power of the Lord. Now Moses truly was sent by him. It’s a sign.
Or think of the Baal worshippers—following the wrong god—when fire is sent from heaven.
And then the whole of the earthly ministry of the Lord Jesus is filled with signs: from the virgin conception (which Isaiah prophesied as a sign) to the turning of water into wine, which John calls his first sign. John is very clear: the signs are recorded so that we may believe that Jesus is the Christ, the Son of God. That is one way that God bears witness to who he is.
