The Angelic Doctor Against the Errors of the Greeks - Part Two - Chapter 7

That he is also the seal of the Son.

Similarly they say that the Spirit is the seal of the Son. For Athanasius in the aforementioned letter says: “The Holy Spirit is the anointing and seal, impressing the image contained in himself, in which Spirit, as truly as in his own seal, that is, by the image of his nature, God the Word seals, and marks the Church his spouse, impressing on her his very own image.” And further on: “Christ in his own image anoints, and super-anoints the Church and through him, as through his seal containing his own essence, he impresses himself on the Church his spouse.”  Chrysostom commenting on Romans says: “If the Spirit is the character and seal of Christ, he who does not have the seal and character of Christ is not Christ’s.” 

It is well known, however, that the character and seal are derived from him whose they are. Hence, from the texts of these Doctors it is expressly shown that the Holy Spirit derives from the Son.

The texts of these Doctors, moreover, are also confirmed by the authority of Holy Scripture. For in 2Cor. 1:22 it is said: God who has anointed us has also put his seal upon us and given his Spirit in our hearts as his guarantee. This is to be understood, naturally, of the Father and Son, since both give us the Holy Spirit, as has been shown.  And in Eph. 1: 13-14: In whom, that is, in Christ, you also, who have heard the word of truth, the gospel of your salvation and have believed in him, were sealed with the promised Holy Spirit, who is the guarantee of our inheritance.