From Pope Pius XII’s encyclical Mystici Corporis Christi (1943):
That this Mystical Body which is the Church should be called Christ’s is proved in the second place from the fact that He must be universally acknowledged as its actual Head. “He,” as St. Paul says, “is the Head of the Body, the Church.”43 He is the Head from whom the whole body perfectly organized, “groweth and maketh increase unto the edifying of itself.”44
You are familiar, Venerable Brethren, with the admirable and luminous language used by the masters of Scholastic Theology and chiefly by the Angelic and Common Doctor, when treating this question; and you know that the reasons advanced by Aquinas are a faithful reflection of the mind and writings of the Holy Fathers, who moreover merely repeated and commented on the inspired word of Sacred Scripture.
However for the good of all We wish to touch on this point briefly. And first of all it is clear that the Son of God and of the Blessed Virgin is to be called the head of the Church by reason of His singular pre-eminence. For the Head is in the highest place. But who is in a higher place than Christ God, who as the Word of the Eternal Father must be acknowledged to be the “firstborn of every creature?”45 Who has reached more lofty heights than Christ Man who, though born of the Immaculate Virgin, is the true and natural Son of God, and in virtue of His miraculous and glorious resurrection, a resurrection triumphant over death, has become the “firstborn of the dead?”46 Who finally has been so exalted as He, who as “the one mediator of God and men”47 has in a most wonderful manner linked earth to heaven, who, raised on the Cross as on a throne of mercy, has drawn all things to Himself,48 who, as the Son of Man chosen from among thousands, is beloved of God beyond all men, all angels and all created things?49
44 Cf. Eph., IV, 16; Col., II, 19.
45 Col., I, 15.
46 Col., I, 18; Apoc., I, 5.
47 I Tim., II, 5.
48 Cf. John, XII, 32.
49 Cf. Cyr. Alex., Comm. in Ioh. I, 4: Migne, Patrologiæ Cursus completus, Series Græca, LXXIII, 69; St. Thos., Summa Theologiæ I, q. 20, a. 4, ad 1.
— Mystici Corporis Christi 34-36