30 And indeed that which we called the fourth statement, that is the manifestation of the Spirit in the bestowal of what is profitable, has a clear meaning. For the Apostle has enumerated the profitable gifts through which this manifestation of the Spirit took place. Now in these diverse activities that Gift is set forth in no uncertain light of which our Lord had spoken to the apostles when He taught them not to depart from Jerusalem; but wait, said He, for the promise of the Father which you heard from My lips: for John indeed baptized with water, but you shall be baptized with the Holy Ghost, which you shall also receive not many days hence. And again: But you shall receive power when the Holy Ghost comes upon you; and you shall be My witnesses in Jerusalem, and in all Judæa, and in Samaria, and unto the uttermost part of the earth. He bids them wait for the promise of the Father of which they had heard from His lips.
We may be sure that here we have a reference to the Father's same promise. Hence it is by these miraculous workings that the manifestation of the Spirit takes place. For the gift of the Spirit is manifest, where wisdom makes utterance and the words of life are heard, and where there is the knowledge that comes of God-given insight, lest after the fashion of beasts through ignorance of God we should fail to know the Author of our life; or by faith in God, lest by not believing the Gospel of God, we should be outside His Gospel; or by the gift of healings, that by the cure of diseases we should bear witness to His grace Who bestows these things; or by the working of miracles, that what we do may be understood to be the power of God, or by prophesy, that through our understanding of doctrine we might be known to be taught of God; or by discerning of spirits, that we should not be unable to tell whether any one speaks with a holy or a perverted spirit; or by kinds of tongues, that the speaking in tongues may be bestowed as a sign of the gift of the Holy Spirit; or by the interpretation of tongues, that the faith of those that hear may not be imperilled through ignorance, since the interpreter of a tongue explains the tongue to those who are ignorant of it. Thus in all these things distributed to each one to profit withal there is the manifestation of the Spirit, the gift of the Spirit being apparent through these marvellous advantages bestowed upon each.
Source: On the Trinity (New Advent)