As opposed to infidel, faithful is one who believes in God and His revelation in Christ, one who has divine, or Christian, faith. Normally this faith is had in the fold of the Catholic Church; but all the baptized who believe in Christ and in good conscience are "outside the Church" (separated brethren) also have the faith. [This is the basis of Catholic participation in ecumenism; cf. Vatican II's Decree on Ecumenism, Unitatis redintegratio 1; Acta Apostolicae Sedis 57 (1964) 90–91.] Nor is it excluded that non-Christians who, without any fault of their own, are ignorant of Christ and His Church should have implicit faith, viz, faith implied in the grace they are given when they follow their conscience in doing all that they know God wants of them (cf. H. Denzinger, Enchiridion symbolorum, ed. A. Schönmetzer [Freiburg 1963] 2866). They may be anonymous Christians, or infidels in appearance only, without having received any private revelation, but not without the grace of faith transforming their "natural belief" in God into divine faith.
In any of these three classes of faithful (Catholics, non-Catholics, and non-Christian believers) faith normally goes together with hope and charity, or the state of grace. The faithful not only believe in Christ; they are also expected to live according to the gospel. Yet faith can exist without the state of grace. It is defined doctrine that when sanctifying grace is lost by grave sin, faith as an infused virtue is not necessarily lost (cf. Enchiridion symbolorum 1578); it is lost only by grave sin against faith. Catholic sinners are still faithful and members of the Church, but members that are critically ill. Although theirs is not a living faith, yet it is a gift of God's grace (cf. Enchiridion symbolorum 3010, 3035). Theology endeavors to show how it is possible for the infused habit of faith to remain without sanctifying grace and charity; a man can freely assent to God revealing without yet living in accordance with this belief (Virtue is not mere knowledge; it requires, besides, good will and effort), but this "dead faith" entails a division of the will, the sinner partly obeying God by believing and partly disobeying God by not doing what He demands. This places his faith in an abnormal and possibly precarious condition.
Because faith is a supernatural gift of God that requires one's free cooperation, the faithful who have received the gift of faith must cooperate with that grace. They need not be making acts of faith all the time—this is not possible—but they ought to make such acts whenever required by their Christian duty. Besides, they must take care to have an enlightened faith and to acquire the knowledge of their religion that befits their state in life. Their cooperation with the grace of faith will be wholehearted only when they live in accordance with their belief, i.e., in a state of grace, so that their faith is living, not dead, faith. Nor should they omit to pray for the grace of perseverance, that will help them to make their minds and wills ever more steadfast in the free assent to divine revelation.
See Also: virtue.
Bibliography: s. harent, Dictionnaire de théologie catholique, ed. a. vacant et al., (Paris 1903–50) 6.1:357–393, j. trÜtsch, Lexikon für Theologie und Kirche, ed. j. hofer and k. rahner (Freiberg 1957–65) 4:920–925. g. jacquemet, Catholicisme 4:1262. j. duplacy, ibid. 1269–75.
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"Faithful." New Catholic Encyclopedia. . Encyclopedia.com. (September 25, 2018). http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/faithful
"Faithful." New Catholic Encyclopedia. . Retrieved September 25, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/faithful