Reply to Objection 6: It is not necessary that the invisible mission should always be made manifest by some visible external sign; but, as is said (1 Cor. 12:7)---"the manifestation of the Spirit is given to every man unto profit"---that is, of the Church. This utility consists in the confirmation and propagation of the faith by such visible signs. This has been done chiefly by Christ and by the apostles, according to Heb. 2:3, "which having begun to be declared by the Lord, was confirmed unto us by them that heard."
Thus in a special sense, a mission of the Holy Ghost was directed to Christ, to the apostles, and to some of the early saints on whom the Church was in a way founded; in such a manner, however, that the visible mission made to Christ should show forth the invisible mission made to Him, not at that particular time, but at the first moment of His conception. The visible mission was directed to Christ at the time of His baptism by the figure of a dove, a fruitful animal, to show forth in Christ the authority of the giver of grace by spiritual regeneration; hence the Father's voice spoke, "This is My beloved Son" (Mt. 3:17), that others might be regenerated to the likeness of the only Begotten. The Transfiguration showed it forth in the appearance of a bright cloud, to show the exuberance of doctrine; and hence it was said, "Hear ye Him" (Mt. 17:5). To the apostles the mission was directed in the form of breathing to show forth the power of their ministry in the dispensation of the sacraments; and hence it was said, "Whose sins you shall forgive, they are forgiven" (Jn. 20:23): and again under the sign of fiery tongues to show forth the office of teaching; whence it is said that, "they began to speak with divers tongues" (Acts 2:4). The visible mission of the Holy Ghost was fittingly not sent to the fathers of the Old Testament, because the visible mission of the Son was to be accomplished before that of the Holy Ghost; since the Holy Ghost manifests the Son, as the Son manifests the Father. Visible apparitions of the divine persons were, however, given to the Fathers of the Old Testament which, indeed, cannot be called visible missions; because, according to Augustine (De Trin. ii, 17), they were not sent to designate the indwelling of the divine person by grace, but for the manifestation of something else.