28 There is therefore in us a certain learned ignorance, so to speak— an ignorance which we learn from that Spirit of God who helps our infirmities. For after the apostle said, “If we hope for that we see not, then do we with patience wait for it,” he added in the same passage, “Likewise the Spirit also helps our infirmities: for we know not what we should pray for as we ought, but the Spirit itself makes intercession for us, with groanings which cannot be uttered. And He that searches the hearts knows what is in the mind of the Spirit, because He makes intercession for the saints according to the will of God.” This is not to be understood as if it meant that the Holy Spirit of God, who is in the Trinity, God unchangeable, and is one God with the Father and the Son, intercedes for the saints like one who is not a divine person; for it is said, “He makes intercession for the saints,” because He enables the saints to make intercession, as in another place it is said, “The Lord your God proves you, that He may know whether ye love Him,” i.e. that He may make you know.
He therefore makes the saints intercede with groanings which cannot be uttered, when He inspires them with longings for that great blessing, as yet unknown, for which we patiently wait. For how is that which is desired set forth in language if it be unknown, for if it were utterly unknown it would not be desired; and on the other hand, if it were seen, it would not be desired nor sought for with groanings?
Source: Letters (New Advent)