“In those days Jesus came from Nazareth in Galilee and was baptized by John in the Jordan. Immediately coming up out of the water, He saw the heavens opening, and the Spirit like a dove descending upon Him; and a voice came out of the heavens: “You are My beloved Son, in You I am well-pleased” (Mark 1:9-11).
Here we see God the Father, God the Son, God the Holy Spirit all together in one place. This is significant because it teaches us a bit about the nature of the Trinity. Jesus is no Bruce Wayne–wealthy business man by day, but Batman by night. Jesus is no Clark Kent–mild mannered reporter by day, buy Superman by night. Such is the heresy of modalism, which says that God the Father is God the Son is God the Spirit, just in different forms. (This doctrine, by the way, is alive and well today. Steer clear from it.)
But we see something different here. We see the Father, the Son, and the Holy Spirit, all present in the same place at the same time. We believe in One God, and three persons of the Godhead.