Final answer:
John the Baptist was told by God the Father that Jesus was the one who would baptize with the Holy Spirit when he witnessed the Spirit descending upon Jesus and remaining on him.
Step-by-step explanation:
John the Baptist was informed that the One upon whom he saw the Spirit descending and remaining is He who baptizes with the Holy Spirit. This revelation was provided to him by God the Father. This event is significant as it not only signifies the anointing of Jesus Christ by the Holy Spirit but also denotes Jesus as the one who will baptize with the Holy Spirit, a pivotal aspect of Christian belief.
In the accounts given in the Gospels of Matthew, Mark, and Luke, it is described that during the baptism of Jesus, the heavens opened, and the Holy Spirit descended in the form of a dove upon him, with a voice from heaven saying, "This is my beloved Son, in whom I am well pleased." This voice is the way God the Father indicated to John the Baptist that Jesus was the appointed one.