Most Protestant Evangelicals hold that a Christian is “once saved always saved,” by which they mean that once a person has committed his life to Christ, he can never do anything to undo this gift of salvation.
The Protestant explanation assumes that since the gift of salvation was freely granted, there is nothing that one can do to lose it. Contrary to this, we know that gifts can be abandoned, rejected, or destroyed by the ill will or neglect of the recipient. A father may purchase a sports car and freely give it to his son as a gift. It is rightly assumed that a gift cannot be “ungifted” or taken away. I am sure that the son would receive the car gleefully. However, the son may turn around and sell the car for drugs, crash the car, or neglect the car so that it no longer functions. The gift was not “ungifted.” Rather, the worth of the gift was rejected through negligence.
Catholicism views salvation in a similar way—a gift is given freely, but it may be spurned and squandered. God never "ungifts" salvation, but humans do reject, squander, wreck this gift. The Father freely gave His Son Jesus Christ to suffer, die, and rise again for our salvation. Through the Holy Spirit, God pours out this love upon us so that we become the children of God. In the words of Saint Paul, we become earthen vessels containing inestimable treasure (1 Cor 4:7). And this salvation is a gift.
If we trash the gift and abandon the twofold charity of love for God and neighbor through mortal sin - it's not God's fault. We are the one's who squandered it, not God. There is no lack God's love. So let us cherish the gift of grace and the gift of the true faith. Even if we've made mistakes before, we can pray for an increase of grace so as to keep our lives pure and lovely for the God who has literally done everything to save us.
Sacred Heart of Jesus, have mercy on us.