As to know the Father and the Son is life, so is it life to know the Holy Spirit; and therefore in the Godhead He is not to be separated from the Father.
26. Let them say, then, wherein they think that there is an unlikeness in the divine operation. Since as to know the Father and the Son is life, as the Lord Himself declared, saying: “This is life eternal to know You the only true God, and Jesus Christ, Whom You have sent,” so, too, to know the Holy Spirit is life. For the Lord said: “If you love Me, keep My commandments, and I will ask the Father and He shall give you another Paraclete, that He may abide with you for ever, even the Spirit of Truth, Whom the world cannot receive, because it sees Him not, neither knows Him, but you know Him, for He is with you, and in you.”
27. So, then, the world had not eternal life, because it had not received the Spirit; for where the Spirit is, there is eternal life; for the Spirit Himself it is Who effects eternal life. Wherefore I wonder why the Arians stir the question as to the only true God. For as it is eternal life to know the only true God, so, too, is it eternal life to know Jesus Christ; so, again, it is eternal life to know the Holy Spirit, Whom, as also the Father, the world does not see, and, as also the Son, does not know. But he who is not of this world has eternal life, and the Spirit, Who is the Light of eternal life, remains with him for ever.
28. If the knowledge of the only true God confers the same benefit as the knowledge of the Son and of the Spirit, why do you sever the Son and the Spirit from the honour of the true God, when you do not sever Him from conferring the benefit? For of necessity you must either believe that this is the greatest gift of the only true Godhead, and will confess the only true Godhead as of the Father, so also of the Son and of the Holy Spirit; or if you say that he, too, can give life eternal who is not true God, it will happen that you derogate rather from the Father, Whose work you do not consider to be the chief work of the only true Godhead, but one to be compared to the work of a creature.
Source: On the Holy Spirit (New Advent)