This admirable faith of the Roman centurion, so highly praised by our Lord Himself, has been considered throughout the centuries as a model for all true believers.
Faith is a supernatural, theological virtue by which, relying on the authority of God, we firmly believe whatever God has revealed and the Church proposes for our belief. Faith is called a virtue because it is a habit inclining us to good; it is supernatural, because it is not acquired by our own efforts, but is infused into our souls by God Himself; it is termed theological, because it has God for its immediate object; its motive is the authority of God, because only God, who can neither deceive nor be deceived, is the author of the truths of faith. God delivers His word to us, not directly, but through His infallible Church (Matt. xxviii. 19; Rom. x. 17). Scripture and tradition, without an authoritative interpreter, are not reliable guides in matters of faith, as is proved by the numerous mutually contradicting Protestant sects. Faith differs, from opinion, which is doubtful assent; from knowledge, which rests on experience or reason; from human belief, which depends on the authority of men. The object of faith is not some, but all of the truths that God has revealed and proposes to us through the Church. The Church proposes the teachings of revelation to us mainly in the Apostles' Creed, and in the definitions of the Popes and Councils.
The Apostles' Creed contains the fundamental truths which we are to believe. It is necessary to believe all the truths the Church teaches, but it is not necessary to know them all explicitly. The truths absolutely necessary to be known by all are: that there is a God; that there is a future life of reward and punishment (Heb. xi. 6 ff.). Since the preaching of the Gospel, it is also required to know and believe the mysteries of the Trinity and of the Incarnation and Redemption (John xiv. 6; xvii. 33). Anyone ignorant of these essential truths cannot be absolved in confession. Parents should instruct their children in these important doctrines from their earliest years. Truths that all are bound to know, as far as they are able are: the Articles of the Creed; the Commandments; the Sacraments, at least those that a given person needs to receive; the Lord's Prayer, the acts of the various virtues, such as, the acts of faith, hope, charity, contrition.
Faith must be firm, i.e., it must exclude all doubt, hesitation, or disbelief. It must be blind, i.e., we must not seek any other reason than the authority of God for what we are asked to believe (John xx. 29). The reason for this latter quality is that faith, being concerned with supernatural truths, is above the natural grasp of our finite minds. The fact that we cannot understand the truths of faith is no reason for rejecting them; because they have God for their author; because the natural world is filled with mysteries which we accept but cannot understand; because constantly we blindly trust the authority of scientists, historians, doctors, lawyers, etc. Faith is reasonable, i.,e., we can establish on rational grounds all the foundations of faith, namely, the existence, knowledge, and truthfulness of God, the divinity of Christ and of the mission of the Church. Faith should be entire, i.e., it must extend to every dogma without exception.
Faith is necessary for salvation (Heb. xi. 6), for it is the root and foundation of our justification. Without faith it is impossible to perform works that are meritorious of life eternal, although one may do many things that are naturally good. Faith imposes a two-fold obligation: a negative obligation, which always binds, of never sinning against it; a positive obligation of making acts of faith before God and of professing our faith before men. Sins against faith are: infidelity, i.e., the total rejection of the Christian religion by those who remain outside the true faith, although they know it sufficiently well to embrace it; apostasy, i.e., the rejection of the Christian religion for a false one; heresy, i.e., the obstinate denial of an Article of faith by a baptized person; deliberate doubt concerning a matter of faith; exposing one's self to the danger of losing the faith by keeping bad company, reading injurious literature, etc. We are obliged to make acts of faith from time to time, especially when in danger of losing our faith. It is never permissible to deny the faith before men, even in appearance (2 Mach. vi. 21 ff.), or by silence; on the contrary, a person is bound to profess his faith publicly whenever God's glory or our neighbor's good requires it (Matt. v. 16).
The knowledge derived from faith is infinitely superior to that which comes from human wisdom, and is at the same time far easier and more secure. How highly then should we appreciate the gift of faith! We should carefully avoid all things by which faith is lost, such as, wilful doubt or denial of Articles of faith; or imperiled, such as, neglect of religious duties, bad books or company, mixed marriages, Godless education, joining secrete societies (ex.Freemasons), etc. We should pray for a great and living faith.