Nevertheless he calls the Spirit, who is of the essence of God the Father, or of his own essence, another advocate. For the principle of the essence is one with regard to both; it does not exclude the Spirit but allows the principle of the difference to be conceived of solely in his being and subsisting in his own person. For the Spirit is not a Son, but we accept by faith that he exists and subsists truly and individually as that which he is. For he is the Spirit of the Father and the Son. But since the Son knew that he himself was in truth an advocate, and is named as such in the sacred Scriptures, he calls the Spirit ‘another advocate’, not that the Spirit who is and is said to be the Spirit of the Son, can by any chance bring about anything of a different kind in the saints than he can himself. That the Son is himslf an advocate, both in name and in reality, John will witness in his own writings, where he says, ‘I am writing this to you so that you may not sin’ but if anyone does sin, we have an advocate with the Father, Jesus Christ the righteous, and he is the expiation for our sins’ (1 Jn 2:1-2). He therefore calls the Spirit ‘another advocate’, intending that he should be conceived of as a seperate hypostasis, yet possessing such a close likeness to himself and with the power to operate in a manner identical to that in which he himslef might perhaps do, that he appears to be the Son himself and not at all different. For he is his Spirit. And indeed he called him the very Spirit of truth, also calling himself the truth in the passage we are considering (Jn 14:6).
~ Cyril of Alexandria on John 9:1