The Lord was Baptized, not to be cleansed Himself, but to cleanse the waters, so that those waters, cleansed by the flesh of Christ which knew no sin, might have the power of Baptism.
Ambrose... the Apostle Paul says that we are sealed in the Spirit (cf. Eph. 1:13); since we have in the Son the image of the Father, and in the Spirit the seal of the Son. Let us, then, sealed by this Trinity, take more diligent heed, lest either levity of character or the deceit of any unfaithfulness unseal the pledge which we have received in our hearts.
Ambrose