Matthew 3:13-17:
13Then cometh Jesus from Galilee to Jordan unto John, to be baptized of him. 14But John forbad him, saying, I have need to be baptized of thee, and comest thou to me? 15And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness. Then he suffered him. 16And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: 17And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.
In this revelation, God has differentiated Himself: the Father has the voice; the Son, the human nature; and the Holy Spirit, the form of a dove. These are things widely different from one another, not only in appearance but also in kind. When reason intervenes, it wants to judge, but mathematics cannot deal with it; the matter is too high. So close your eyes, turn off your reason, and say, “God has said it.”
From this account in Matthew’s Gospel you should learn that the event that took place once visibly will not cease until the Last Day when God gathers us into His flock. The text says clearly that John the Baptist saw with his eyes, but the same thing still happens every day, thought it has to be seen with the eye of faith. Heaven was opened back then, and it will not close again until the Last Day. So in the present day heaven is still open over the entire world. Note well that this event has not ended—as one might say of the histories of David (or the crossing of the Red Sea) and others: “This is over and done with.” Say, rather: “Ever since that time heaven stands open.” If you see that the Baptism of an infant, the Sacrament, Absolution, and the ministry of the Word are administered in accordance with the kingdom of Christ, then say, “Heaven is open, the voice of the Father resounds, the Holy Spirit hovers, the Son stands in the water.” If heaven were closed, who would want to baptize, preach, administer the Sacrament, absolve? This realm is called the kingdom of heaven, for Christ has opened up heaven and the Holy Spirit hovers and the Father says “My vineyeard is before me,” etc. [Song of Solomon 8:12]. This is so that we Christians can live more uprightly and honorably, that we might always preach this, administer the Sacraments, and think: there sits the Father, saying, “Listen to Him” [Matt. 17:5].
From “Sermon for the First Sunday After Epiphany, Matthew 3:13-17, Preached in Wittenberg, January 13, 1544” by Martin Luther (Vol. 58, pp. 73-74, Luther’s Works, Concordia Publishing House, St. Louis, 2010).