By natural reason man can know God with certainty, on the basis of his works. But there is another order of knowledge, which man cannot possibly arrive at by his own powers: the order of divine Revelation. Through an utterly free decision, God has revealed himself and given himself to man. This he does by revealing the mystery, his plan of loving goodness, formed from all eternity in Christ, for the benefit of all men. God has fully revealed this plan by sending us his beloved Son, our Lord Jesus Christ, and the Holy Spirit.
Source: Catechism of the Catholic Church (Vatican.va)