By His Revelation, "the invisible God, from the fullness of His love, addresses men as His friends, and moves among them, in order to invite and receive them into His own company." The adequate response to this invitation is faith.
By faith, man completely submits his intellect and his will to God. With his whole being man gives his assent to God the revealer. Sacred Scripture calls this human response to God, the author of revelation, "the obedience of faith".
In faith, the human intellect and will cooperate with divine grace: "Believing is an act of the intellect assenting to the divine truth by command of the will moved by God through grace." (St. Thomas Aquinas)
"Faith seeks understanding" (St. Anselm): it is intrinsic to faith that a believer desires to know better the One in whom he has put his faith, and to understand better what He has revealed; a more penetrating knowledge will in turn call forth a greater faith, increasingly set afire by love. The grace of faith opens "the eyes of your hearts"(Eph 1:18) to a lively understanding of the contents of Revelation: that is, of the totality of God's plan and the mysteries of faith, of their connection with each other and with Christ, the center of the revealed mystery. "The same Holy Spirit constantly perfects faith by His gifts, so that Revelation may be more and more profoundly understood." In the words of St. Augustine, "I believe, in order to understand; and I understand, the better to believe."
The disciple of Christ must not only keep the faith and live on it, but also profess it, confidently bear witness to it, and spread it: "All however must be prepared to confess Christ before men and to follow him along the way of the Cross, amidst the persecutions which the Church never lacks." Service of and witness to the faith are necessary for salvation: "So every one who acknowledges me before men, I also will acknowledge before my Father who is in heaven; but whoever denies me before men, I also will deny before my Father who is in heaven." (mT. 10:32-33)
The Catechism of the Catholic Church is available online...