... 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.
AmbroseOn consideration.. .of the reason wherefore men have so far gone astray, or that many - alas! - should follow diverse ways of belief concerning the Son of God, the marvel seems to be, not at all that human knowledge has been baffled in dealing with superhuman things, but that it has not submitted to the authority of the Scriptures
Ambrose