29 Now that He is the Spirit of the Father, the Son Himself says, “He proceeds from the Father.” And in another place, “For it is not ye that speak, but the Spirit of your Father which speaks in you.” And that He is the Spirit of the Son also the Apostle says, “God has sent the Spirit of His Son into your hearts, crying, Abba Father;” that is, making you cry. For it is we that cry; but in Him, that is, by His shedding abroad love in our hearts, without which whoso cries, cries in vain. Whence he says again, “If any man have not the Spirit of Christ, he is none of His.” To which Person then in the Trinity could the communion of this fellowship peculiarly appertain, but to that Spirit which is common to the Father and the Son?
Source: Sermons on Selected Lessons of the New Testament (New Advent)