13 It follows in the Creed, “And in the Holy Ghost.” This Trinity, one God, one nature, one substance, one power; highest equality, no division, no diversity, perpetual dearness of love. Would ye know the Holy Ghost, that He is God? Be baptized, and you will be His temple. The Apostle says, “Do you not know that your bodies are the temple within you of the Holy Ghost, Whom you have of God?” A temple is for God: thus also Solomon, king and prophet, was bidden to build a temple for God.
If he had built a temple for the sun or moon or some star or some angel, would not God condemn him? Because therefore he built a temple for God he showed that he worshipped God. And of what did he build? Of wood and stone, because God deigned to make unto Himself by His servant an house on earth, where He might be asked, where He might be had in mind. Of which blessed Stephen says, “Solomon built Him a house; howbeit the Most High dwells not in temples made by hand.” If then our bodies are the temple of the Holy Ghost, what manner of God is it that built a temple for the Holy Ghost?
But it was God. For if our bodies be a temple of the Holy Ghost, the same built this temple for the Holy Ghost, that built our bodies. Listen to the Apostle saying, “God has tempered the body, giving unto that which lacked the greater honor;” when he was speaking of the different members that there should be no schisms in the body. God created our body. The grass, God created; our body Who created? How do we prove that the grass is God's creating? He that clothes, the same creates.
Read the Gospel, “If then the grass of the fields,” says it, “which today is, and tomorrow is cast into the oven, God so clothes.” He, then, creates Who clothes. And the Apostle: “You fool, that which you sow is not quickened except it die; and that which you sow, you sow not that body that shall be, but a bare grain, as perchance of wheat, or of some other grain; but God gives it a body as He would, and to each one of seeds its proper body.” If then it be God that builds our bodies, God that builds our members, and our bodies are the temple of the Holy Ghost, doubt not that the Holy Ghost is God. And do not add as it were a third God; because Father and Son and Holy Ghost is One God. So believe ye.
Source: On the Creed: A Sermon to Catechumens (New Advent)