23 For I am aware that the Son of God is revealed under the title Spirit of God in order that we may understand the presence of the Father in Him, and that the term Spirit of God may be employed to indicate Either, and that this is shown not only on the authority of prophets but of evangelists also, when it is said, The Spirit of the Lord is upon Me; therefore He has anointed Me. And again, Behold My Servant Whom I have chosen, My beloved in Whom My soul is well pleased, I will put My Spirit upon Him. And when the Lord Himself bears witness of Himself, But if I in the Spirit of God cast out devils, then has the kingdom of God come upon you. For the passages seem without any doubt to denote either Father or Son, while they yet manifest the excellence of nature.
Source: On the Trinity (New Advent)