To be human, “man’s response to God by faith must be free, and… therefore nobody is to be forced to embrace the faith against his will. the act of faith is of its very nature a free act.”God calls men to serve him in spirit and in truth. Consequently they are bound to him in conscience, but not coerced. . . This fact received its fullest manifestation in Christ Jesus.” Indeed, Christ invited people to faith and conversion, but never coerced them. “For he bore witness to the truth but refused to use force to impose it on those who spoke against it. His kingdom… grows by the love with which Christ, lifted up on the cross, draws men to himself.”
Believing in Jesus Christ and in the One who sent him for our salvation is necessary for obtaining that salvation. “Since “without faith it is impossible to please (God) ” and to attain to the fellowship of his sons, therefore without faith no one has ever attained justification, nor will anyone obtain eternal life ‘But he who endures to the end.”
Faith is an entirely free gift that God makes to man. We can lose this priceless gift, as St. Paul indicated to St. Timothy: “Wage the good warfare, holding faith and a good conscience. By rejecting conscience, certain persons have made shipwreck of their faith.” To live, grow and persevere in the faith until the end we must nourish it with the word of God; we must beg the Lord to increase our faith; it must be “working through charity,” abounding in hope, and rooted in the faith of the Church.
From the Catechism of the Catholic Church 160-162