From Pope Pius XII’s encyclical Mystici Corporis Christi (1943):
If we consider closely all these mysteries of the Cross, those words of the Apostle are no longer obscure, in which he teaches the Ephesians that Christ, by His blood, made the Jews and Gentiles one “breaking down the middle wall of partition . . . in his flesh” by which the two peoples were divided; and that He made the Old Law void “that He might make the two in Himself into one new man,” that is, the Church, and might reconcile both to God in one Body by the Cross.”40
The Church which He founded by His Blood, He strengthened on the Day of Pentecost by a special power, given from heaven. For, having solemnly installed in his exalted office him whom He had already nominated as His Vicar, He had ascended into Heaven; and sitting now at the right hand of the Father He wished to make known and proclaim His Spouse through the visible coming of the Holy Spirit with the sound of a mighty wind and tongues of fire.41 For just as He Himself when He began to preach was made known by His Eternal Father through the Holy Spirit descending and remaining on Him in the form of a dove,42 so likewise, as the Apostles were about to enter upon their ministry of preaching, Christ our Lord sent the Holy Spirit down from Heaven, to touch them with tongues of fire and to point out, as by the finger of God, the supernatural mission and office of the Church.
40 Cf. Eph., II, 14-16.
41 Cf. Acts, II, 1-4.
42 Cf. Luke, III, 22; Mark, I, 10.
— Mystici Corporis Christi 32-33