Faith is a personal adherence of the whole man to God who reveals himself. It involves an assent of the intellect and will to the self-revelation God has made through his deeds and words. "To believe" has thus a twofold reference: to the person, and to the truth: to the truth, by trust in the person who bears witness to it.
The faithful must believe in no one but God: the Father, the Son and the Holy Spirit.
Faith is a supernatural gift from God. In order to believe, man needs the interior helps of the Holy Spirit. "Believing" is a human act, conscious and free, corresponding to the dignity of the human person. "Believing" is an ecclesial act. the Church's faith precedes, engenders, supports and nourishes our faith. the Church is the mother of all believers. "No one can have God as Father who does not have the Church as Mother" (St. Cyprian, De unit. 6: PL 4, 519).
We believe all "that which is contained in the word of God, written or handed down, and which the Church proposes for belief as divinely revealed" (Paul VI, CPG # 20). Faith is necessary for salvation. the Lord himself affirms: "He who believes and is baptised will be saved; but he who does not believe will be condemned" (⇒ Mk 16:16). "Faith is a foretaste of the knowledge that will make us blessed in the life to come" (St. Thomas Aquinas. Comp. theol. 1, 2).