The baptizing of Christ

"CHRIST wished to be baptized in order to consecrate the baptism wherewith we were to be baptized. And therefore it behooved those things to be shown forth which belong to the efficacy of our baptism: concerning which efficacy three points are to be considered. First, the principal power from which it is derived; and this, indeed, is a heavenly power. For which reason, when Christ was baptized, heaven was opened, to show that in future the heavenly power would sanctify baptism."

~St. Thomas Aquinas: Summa Theologica, III, Q. 39, A. 5.
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Baptism of Christ, by Donatello. 

Marble, after 1425; Duomo, Arezzo