Objection 1. It seems that it is not proper to Christ to be Head of the Church. For it is written (1 Samuel 15:17): "When thou wast a little one in thy own eyes, wast thou not made the head of the tribes of Israel?" Now there is but one Church in the New and the Old Testament. Therefore it seems that with equal reason any other man than Christ might be head of the Church.
Objection 2. Further, Christ is called Head of the Church from His bestowing grace on the Church's members. But it belongs to others also to grant grace to others, according to Ephesians 4:29: "Let no evil speech proceed from your mouth; but that which is good to the edification of faith, that it may administer grace to the hearers." Therefore it seems to belong also to others than Christ to be head of the Church.
Objection 3. Further, Christ by His ruling over the Church is not only called "Head," but also "Shepherd" and "Foundation." Now Christ did not retain for Himself alone the name of Shepherd, according to 1 Peter 5:4, "And when the prince of pastors shall appear, you shall receive a never-fading crown of glory"; nor the name of Foundation, according to Apocalypse 21:14: "And the wall of the city had twelve foundations." Therefore it seems that He did not retain the name of Head for Himself alone.
On the contrary, It is written (Colossians 2:19): "The head" of the Church is that "from which the whole body, by joints and bands being supplied with nourishment and compacted groweth unto the increase of God." But this belongs only to Christ. Therefore Christ alone is Head of the Church.
I answer that, The head influences the other members in two ways. First, by a certain intrinsic influence, inasmuch as motive and sensitive force flow from the head to the other members; secondly, by a certain exterior guidance, inasmuch as by sight and the senses, which are rooted in the head, man is guided in his exterior acts. Now the interior influx of grace is from no one save Christ, Whose manhood, through its union with the Godhead, has the power of justifying; but the influence over the members of the Church, as regards their exterior guidance, can belong to others; and in this way others may be called heads of the Church, according to Amos 6:1, "Ye great men, heads of the people"; differently, however, from Christ. First, inasmuch as Christ is the Head of all who pertain to the Church in every place and time and state; but all other men are called heads with reference to certain special places, as bishops of their Churches. Or with reference to a determined time as the Pope is the head of the whole Church, viz. during the time of his Pontificate, and with reference to a determined state, inasmuch as they are in the state of wayfarers. Secondly, because Christ is the Head of the Church by His own power and authority; while others are called heads, as taking Christ's place, according to 2 Corinthians 2:10, "For what I have pardoned, if I have pardoned anything, for your sakes I have done it in the person of Christ," and 2 Corinthians 5:20, "For Christ therefore we are ambassadors, God, as it were, exhorting by us."
Reply to Objection 1. The word "head" is employed in that passage in regard to exterior government; as a king is said to be the head of his kingdom.
Reply to Objection 2. Man does not distribute grace by interior influx, but by exteriorly persuading to the effects of grace.
Reply to Objection 3. As Augustine says (Tract. xlvi in Joan.): "If the rulers of the Church are Shepherds, how is there one Shepherd, except that all these are members of one Shepherd?" So likewise others may be called foundations and heads, inasmuch as they are members of the one Head and Foundation. Nevertheless, as Augustine says (Tract. xlvii), "He gave to His members to be shepherds; yet none of us calleth himself the Door. He kept this for Himself alone." And this because by door is implied the principal authority, inasmuch as it is by the door that all enter the house; and it is Christ alone by "Whom also we have access... into this grace, wherein we stand" (Romans 5:2); but by the other names above-mentioned there may be implied not merely the principal but also the secondary authority.
Source: Summa Theologica (New Advent)