154 These things are narrow and paltry, even between men, who for the most part, although they do not agree in action, yet agree in will. So that a certain person being asked what a friend is, answered, “A second self.” If, then, a man so defined a friend as to say, he was a second self, that is to say, through a oneness of love and good-will, how much more ought we to esteem the oneness of Majesty, in the Father, the Son, and the Holy Ghost, when by the same operation and divine power, either the unity, or certainly that which is more, the ταυτότης, as it is called in Greek, is expressed, for ταύτο signifies “the same,” so that the Father, the Son, and the Holy Spirit have the same; so that to have the same will and the same power does not arise from the affection of the will, but inheres in the substance of the Trinity.
155. This is the inheritance of apostolic faith and devotion, which one may observe also in the Acts of the Apostles. Therefore Paul and Barnabas obeyed the commands of the Holy Spirit. And all the apostles obeyed, and immediately ordained those whom the Spirit had ordered to be separated: “Separate Me,” said He, “Barnabas and Saul.” Do you see the authority of Him Who commands? Consider the merit of those who obey.
156. Paul believed, and because he believed he cast off the zeal of a persecutor, and gained a crown of righteousness. He believed who used to make havoc of the Churches; but being converted to the faith, he preached in the Spirit that which the Spirit commanded. The Spirit anointed His champion, and having shaken off the dust of unbelief, presented him as an insuperable conqueror of the unbelievers to various assemblies of the ungodly, and trained him by many sufferings for the prize of his high calling in Christ Jesus.
157. Barnabas also believed, and obeyed because he believed. Therefore, being chosen by the authority of the Holy Spirit, Which came on him abundantly, as a special sign of his merits, he was not unworthy of so great a fellowship. For one grace shone in these whom one Spirit had chosen.
158. Nor was Paul inferior to Peter, though the latter was the foundation of the Church, and the former a wise builder knowing how to make firm the footsteps of the nations who believed; Paul was not, I say, unworthy of the fellowship of the apostles, but is easily comparable with the first, and second to none. For he who knows not that he is inferior makes himself equal.
Source: On the Holy Spirit (New Advent)