Hence it has always been, from the beginning, absolutely necessary for salvation to know, by divine faith, God as the Creator of heaven and earth and the eternal Rewarder of the good and the wicked, and the Incarnation of the Son of God, and consequently the mystery of the Most Holy Trinity; "For he that cometh to God," says St. Paul, "must believe that he is, and is a rewarder of those who seek him." (Heb. xi. 6.) Upon these words of the great Apostle, Cornelius a Lapide comments as follows:
"The knowledge of God acquired from the contemplation of the world teaches only that God is the Author of the world and of all natural blessings, and that only these natural goods can be obtained and asked of him. But God wishes to be honored and loved by men, not only as the Author of natural goods, but also as the Author of the supernatural and everlasting goods in the world to come; and no one can in any other way come to him and to his friendship, please him, and be acceptable to him. Hence true, divine faith is necessary, because it is only by the light of divine faith that we know God, not only as the Author of nature, but also as the Author of grace and eternal glory; and therefore the Apostle says that to know that there is a God, who rewards the good and punishes the wicked, is to know him as such, not only from natural knowledge, and belief, but also from supernatural knowledge and divine faith.
"But if St. Paul speaks here only of these two great truths, it does by no means follow, that he wishes to teach that the supernatural knowledge of these two truths only and divine faith in them are sufficient to obtain justification, that is, to obtain the grace to become the children of God; but they are necessary in order to be greatly animated with hope in undergoing hard labors and struggles for the sake of virtue. However, to obtain the grace of justification, we must also believe other supernatural truths, especially the mystery of the Incarnation of Christ and that of the Most Holy Trinity." (Comm. in Ep. ad Heb., ix. 6.)
"Some theologians," says St. Alphonsus, "hold that the belief of the two other articles - the Incarnation of the Son of God, and the Trinity of Persons - is strictly commanded but not necessary, as a means without which salvation is impossible; so that a person inculpably ignorant of them may be saved. But according to the more common and truer opinion, the explicit belief of these articles is necessary as a means without which no adult can be saved." (First Command. No. 8.) According to St. Augustine (De Praedest. Sanctorum C. 15.) and other Theologians, the predestination, election, and Incarnation of Christ alone were owing, not to the foreseen merit of any one, not even to that of Christ himself, but only to the good pleasure of God. However, the predestination of all men in general, or the election of some in preference to others, is all owing to the merit of Christ, on account of which God has called all men to life everlasting and gives them sufficient grace to obtain it, if they make a proper use of his grace, especially that of prayer.
"That faith," says the same great Doctor of the Church , "is sound, by which we believe that neither any adult nor infant could be delivered from sin and the death of the soul, except by Jesus Christ, the only Mediator between God and man." ( Ep. 190, olim 157, parum a principio.) Hence St Thomas says: Almighty God decreed from all eternity the mystery of the Incarnation, in order that men might obtain salvation through Christ. It was therefore necessary at all times, that this mystery of the Incarnation should, in some manner, be explicitly believed. Undoubtedly, that means is necessarily a truth of faith, by which man obtains salvation. Now men obtain salvation by the mystery of the Incarnation and Passion of Christ; for it is said in the Holy Scripture: "There is no other name under heaven given to men whereby we must be saved." (Acts, iv. 10.) Hence it was necessary at all times that the mystery of the Incarnation of Christ should be believed by all men in some manner (aliqualiter, either implicitly or explicitly), however, in a different way, according to the circumstances, of times and persons.
Before the fall, man believed explicitly the Incarnation of Christ. Ante statum peccati homo habuit explicitam fidem de Christi incarnatione, secundum quod ordinabatur ad consummationem gloriae, non autem secundum quod ordinabatur ad liberationem a peccato per passionem et resurrectionem, quia homo non fuit praescius peccati futuri. But that he had the knowledge of Christ's Incarnation seems to follow from his words: "Wherefore a man shall leave father and mother, and shall cleave to his wife." (Gen. 11. 24) And St. Paul calls this a great sacrament in Christ and in the Church; (Eph. v. 32.) and therefore it cannot be believed that the first man was ignorant of this sacrament.
After the fall of man, the mystery of the Incarnation of Christ was explicitly believed, that is, not only, the Incarnation itself, but also the Passion and Resurrection of Christ, by which mankind is delivered from sin and death; for otherwise they could not have prefigured Christ's Passion by certain sacrifices offered as well before as also after the Written Law, the meaning of which sacrifices was well known to those whose duty it was to teach the religion of God; but as to the rest of the people, who believed that those sacrifices were ordained by God to foreshadow Christ to come, they had thus implicit faith in Christ.
[St. Thomas said:] As the mystery of the Incarnation was believed from the beginning, so, also, was it necessary to believe the mystery of the Most Holy Trinity; for the mystery of the Incarnation cannot be explicitly believed without faith in the Most Holy Trinity, because the mystery of the Incarnation teaches that the Son of God took to himself a human body and soul by the power of the Holy Ghost. Hence, as the mystery of the Incarnation was explicitly believed by the teachers of religion, and implicitly by the rest of the people, so, also, was the mystery of the Most Holy Trinity explicitly believed by the teachers of religion and implicitly by the rest of the people. But in the New Law it must be explicitly believed by all." (De Fide, Q ii., art. vii. et viii.)
God revealed these great truths of salvation to our first parents immediately after the fall. He preserved the knowledge of them through the holy patriarchs and prophets who, in clear language, foretold that the Redeemer would come, and "be a priest upon his throne" (Zach. vi. 13.), "a priest according to the order of Melchisedech," (Ps. cix. 4.), and that he himself would be the victim offered up for the sins of mankind.[The same text of St. Thomas in slightly different wording: pg 289 of Catechism of Perseverance ]