So far as the second thing is concerned [involved in Matt. 16 : 16-18], that is, faith, which is touched upon in the words, "Thou art Christ, the Son of the living God" — it is to be noted that faith is now taken for the act of believing by which we believe, now for the inward state or disposition — habitus — of believing through which we believe, and now for the truth which we believe, as Augustine lays down, de Trinitate, 13 [Nic. Fathers, 3 : 166 sqq.].
In the second place, it is to be noted that there is one faith which is the explicit belief of a faithful man and that there is another faith which is implicit faith as the catholic, who has the disposition — habitus — of faith infused or explicitly acquired, believes in the catholic church in common with others and by reason of that common faith believes implicitly whatever single thing is included under holy mother church. Likewise in believing whatsoever Christ wished to be believed about himself and refusing to believe what he did not wish to be believed about himself, he believes every article, affirmative or negative, which is to be believed about Christ. This faith Peter had implicitly when he expressly confessed Christ to be true God and true man, saying: "Thou art Christ, the Son of the living God." And yet the same Peter explicitly set himself against Christ and his Gospel when, after Christ had said, "All ye shall be offended in me this night" [Matt. 26 : 31], he denied and said: "Though all be offended in thee, yet will I never be offended." Thus also many of the faithful in common [that is, as a body] believe implicitly all the truth of Scripture, and when a truth unknown to them is proposed, they search to see if it is laid down in holy Scripture, and if this is shown to be the case they at once acknowledge the sense which the Holy Spirit insists on. Therefore, whoever has in common with others faith formed in love, this suffices for salvation when accompanied with the grace of perseverance. For God, who gave the first faith, will give to his soldier clearer faith, unless he puts some hindrance in the way. For God does not demand of all his children that they should continuously during their sojourn here be in the particular act of thought about any particular point of faith, but it is enough that, putting aside inertia and callousness, they have faith formed as a habit.
Faith, therefore, we must understand, is twofold : the one unformed, which is exercised by the demons who believe and tremble; the other faith formed in love. The latter, accompanied with perseverance, saves, but not the former. Hence with reference to the faith formed in love the words were spoken: "Whosoever believeth in the Son of God, hath eternal life," John 3 : 15. And the Saviour said to Peter, who had that faith and professed it: "Blessed art thou, Simon Bar-Jonah." This faith is the foundation of the other virtues
So in so far as he lacks faith. For, if he were mindful of the penalty to be inflicted on those sinning in that way and fully believed it and had the faith which comes from divine knowledge — wherewith God knows all things clearly and is present with such sinners — then, without doubt, he would not sin mortal sin.
A person may lack faith in three ways: (1) By weakness, and in this way he is lacking who vacillates in believing and does not persist unto death in the defense of faith. (2) He is lacking in faith who firmly believes the many things which are to be believed and yet is lacking in many things to be believed, which unbelieved things are as holes, and thus he has a shield of faith which is full of holes. (3) He is lacking in faith who lacks in the use of this shield; and this happens in this way: that, though he has the firm habit of things to be believed, he nevertheless lacks in acts of meritorious living because of an undisciplined life. These things are referred to in Titus 1 : 16: "They confess that they know God, but in deeds deny him." Every one, therefore, who is lacking in faith in any of these three ways is wanting in the abiding strength of faith.
And we must remember that faith differs from hope: (1) In this, that hope has reference to the future prize to be obtained, but faith concerns the past, namely such things as that God created the world, that Christ was incarnate, etc. And it concerns also the present, as that God is, that the saints are in heaven, and that Christ sits at the right hand of the Father. Faith also concerns the future, as that Christ will come again in judgment; that all who have not arisen at that time will arise in the day of judgment; and that God will finally reward in bliss all the saints who finish this present life in grace. (2) Hope does not reach the knowledge of faith in that which it hopes for, but it rests in a certain middle act between doubt and belief, so that there are many things which are to be set before the faithful to accept which, when the distinction is removed, they should neither doubt, nor grant, nor deny but only hope for. For example, if it were proposed to me, "Thou shalt be saved," I ought not to grant it, for I do not know whether it is true, nor should I deny it, for I do not know whether it is false, nor should I doubt it — but I should hope for it. (3) Faith also differs from hope in this, that hope is only of good which is possible to him who hopeth, but faith is about the evil as well as about the good, for we believe the forgiveness of sin, which is most certainly a good thing for all who are to be saved; and we believe also that the sin of blasphemy will not be forgiven either in this world or in that which is to come.
And for the reason that believing is an act of faith, that is, to put trust w—fidere — therefore know that to believe that which is necessary for a man to secure blessedness is to adhere firmly and without wavering to the truth spoken as by God. For this truth, because of its certitude, a man ought to expose his life to the danger of death. And, in this way, every Christian is expected to believe explicitly and implicitly all the truth which the Holy Spirit has put in Scripture, and in this way a man is not bound to believe the sayings of the saints which are apart from Scripture, nor should he believe papal bulls, except in so far as they speak out of Scripture, or in so far as what they say is founded in Scripture simply. But a man may believe bulls as probable, for both the pope and his curia make mistakes from ignorance of the truth. And, with reference to this ignorance, it can be substantiated that the pope makes mistakes and may be deceived. Lucre deceives the pope, and he is deceived through ignorance. How far, however, faith ought to be placed in the letters of princes, the instruments of notaries, and the descriptions of men, experience, which is the teacher of things, teaches. For she teaches that these three often make mistakes. Of one kind is the faith which is placed in God. He cannot deceive or be deceived; of another is the faith placed in the pope, who rmay deceive and be deceived. Of one kind is the faith placed in holy Scripture; and another, faith in a bull thought out in a human way. For to holy Scripture exception may not be taken, nor may it be gainsaid; but it is proper at times to take exception to bulls and gainsay them when they either commend the unworthy or put them in authority, or savor of avarice, or honor the unrighteous or oppress the innocent, or implicitly contradict the commands or counsels of God.
It is, therefore, plain which faith is the foundation of the church — the faith with which the church is built upon the Rock, Christ Jesus, for it is that by which the church confesses that "Jesus Christ is the Son of the living God." For Peter spoke for all the faithful, when he said: "Thou art the Christ, the Son of the living God." "This is the victory," says John, "which overcometh the world — even our faith. Who is he that overcometh the world but he that believeth that Jesus is the Son of God?" I John 5 : 4.