Objection 1. It would seem that it is not necessary for the salvation of all that they should believe explicitly in the mystery of Christ. For man is not bound to believe explicitly what the angels are ignorant about: since the unfolding of faith is the result of Divine revelation, which reaches man by means of the angels, as stated above (6; I, 111, 1). Now even the angels were in ignorance of the mystery of the Incarnation: hence, according to the commentary of Dionysius (Coel. Hier. vii), it is they who ask (Psalm 23:8): "Who is this king of glory?" and (Isaiah 63:1): "Who is this that cometh from Edom?" Therefore men were not bound to believe explicitly in the mystery of Christ's Incarnation.
Objection 2. Further, it is evident that John the Baptist was one of the teachers, and most nigh to Christ, Who said of him (Matthew 11:11) that "there hath not risen among them that are born of women, a greater than" he. Now John the Baptist does not appear to have known the mystery of Christ explicitly, since he asked Christ (Matthew 11:3): "Art Thou He that art to come, or look we for another?" Therefore even the teachers were not bound to explicit faith in Christ.
Objection 3. Further, many gentiles obtained salvation through the ministry of the angels, as Dionysius states (Coel. Hier. ix). Now it would seem that the gentiles had neither explicit nor implicit faith in Christ, since they received no revelation. Therefore it seems that it was not necessary for the salvation of all to believe explicitly in the mystery of Christ.
On the contrary, Augustine says (De Corr. et Gratia vii; Ep. cxc): "Our faith is sound if we believe that no man, old or young is delivered from the contagion of death and the bonds of sin, except by the one Mediator of God and men, Jesus Christ."
I answer that, As stated above (5; 1, 8), the object of faith includes, properly and directly, that thing through which man obtains beatitude. Now the mystery of Christ's Incarnation and Passion is the way by which men obtain beatitude; for it is written (Acts 4:12): "There is no other name under heaven given to men, whereby we must be saved." Therefore belief of some kind in the mystery of Christ's Incarnation was necessary at all times and for all persons, but this belief differed according to differences of times and persons. The reason of this is that before the state of sin, man believed, explicitly in Christ's Incarnation, in so far as it was intended for the consummation of glory, but not as it was intended to deliver man from sin by the Passion and Resurrection, since man had no foreknowledge of his future sin. He does, however, seem to have had foreknowledge of the Incarnation of Christ, from the fact that he said (Genesis 2:24): "Wherefore a man shall leave father and mother, and shall cleave to his wife," of which the Apostle says (Ephesians 5:32) that "this is a great sacrament... in Christ and the Church," and it is incredible that the first man was ignorant about this sacrament.
But after sin, man believed explicitly in Christ, not only as to the Incarnation, but also as to the Passion and Resurrection, whereby the human race is delivered from sin and death: for they would not, else, have foreshadowed Christ's Passion by certain sacrifices both before and after the Law, the meaning of which sacrifices was known by the learned explicitly, while the simple folk, under the veil of those sacrifices, believed them to be ordained by God in reference to Christ's coming, and thus their knowledge was covered with a veil, so to speak. And, as stated above (Question 1, Article 7), the nearer they were to Christ, the more distinct was their knowledge of Christ's mysteries.
After grace had been revealed, both learned and simple folk are bound to explicit faith in the mysteries of Christ, chiefly as regards those which are observed throughout the Church, and publicly proclaimed, such as the articles which refer to the Incarnation, of which we have spoken above (Question 1, Article 8). As to other minute points in reference to the articles of the Incarnation, men have been bound to believe them more or less explicitly according to each one's state and office.
Reply to Objection 1. The mystery of the Kingdom of God was not entirely hidden from the angels, as Augustine observes (Gen. ad lit. v, 19), yet certain aspects thereof were better known to them when Christ revealed them to them.
Reply to Objection 2. It was not through ignorance that John the Baptist inquired of Christ's advent in the flesh, since he had clearly professed his belief therein, saying: "I saw, and I gave testimony, that this is the Son of God" (John 1:34). Hence he did not say: "Art Thou He that hast come?" but "Art Thou He that art to come?" thus saying about the future, not about the past. Likewise it is not to be believed that he was ignorant of Christ's future Passion, for he had already said (John 1:39): "Behold the Lamb of God, behold Him who taketh away the sins [Vulgate: 'sin'] of the world," thus foretelling His future immolation; and since other prophets had foretold it, as may be seen especially in Isaias 53. We may therefore say with Gregory (Hom. xxvi in Evang.) that he asked this question, being in ignorance as to whether Christ would descend into hell in His own Person. But he did not ignore the fact that the power of Christ's Passion would be extended to those who were detained in Limbo, according to Zechariah 9:11: "Thou also, by the blood of Thy testament hast sent forth Thy prisoners out of the pit, wherein there is no water"; nor was he bound to believe explicitly, before its fulfilment, that Christ was to descend thither Himself.
It may also be replied that, as Ambrose observes in his commentary on Luke 7:19, he made this inquiry, not from doubt or ignorance but from devotion: or again, with Chrysostom (Hom. xxxvi in Matth.), that he inquired, not as though ignorant himself, but because he wished his disciples to be satisfied on that point, through Christ: hence the latter framed His answer so as to instruct the disciples, by pointing to the signs of His works.
Reply to Objection 3. Many of the gentiles received revelations of Christ, as is clear from their predictions. Thus we read (Job 19:25): "I know that my Redeemer liveth." The Sibyl too foretold certain things about Christ, as Augustine states (Contra Faust. xiii, 15). Moreover, we read in the history of the Romans, that at the time of Constantine Augustus and his mother Irene a tomb was discovered, wherein lay a man on whose breast was a golden plate with the inscription: "Christ shall be born of a virgin, and in Him, I believe. O sun, during the lifetime of Irene and Constantine, thou shalt see me again" [Cf. Baron, Annal., A.D. 780. If, however, some were saved without receiving any revelation, they were not saved without faith in a Mediator, for, though they did not believe in Him explicitly, they did, nevertheless, have implicit faith through believing in Divine providence, since they believed that God would deliver mankind in whatever way was pleasing to Him, and according to the revelation of the Spirit to those who knew the truth, as stated in Job 35:11: "Who teacheth us more than the beasts of the earth."
Source: Summa Theologica (New Advent)