The Lord Jesus himself proclaims, 'This is My Body.' Before the blessing of the heavenly words something of another character is spoken of; after consecration it is designated 'body'. He himself speaks of his blood. Before the consecration it is spoken of as something else; after the consecration it is spoken of as 'blood'. And you say, 'Amen', that is, 'It is true.' What the mouth speaks, let the mind within confess; what the tongue utters, let the heart feel.
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