Christ’s redemptive activity finds its apogee in the death of sacrifice on the cross. On this account it is by excellence but not exclusively the efficient cause of our redemption....No one can be just to whom the merits of Christ’s passion have not been communicated. It is a fundamental doctrine of St. Paul that salvation can be acquired only by the grace merited by Christ (Ludwig Ott, Fundamentals of Catholic Dogma (Rockford: Tan, 1974), pp. 185, 190).
Are the sacraments necessary for salvation?
According to the way God has willed that we be saved the sacraments are necessary for salvation
(John Hardon, The Question and Answer Catholic Catechism (Garden City: Image, 1981), Questions # 401, 402, 461, 462, 1119).
If any one saith that the sacraments of the New Law are not necessary unto salvation...and that without them, or without the desire thereof, men obtain from God, through faith alone, the grace of justification...let him be anathema (The Canons and Decrees of the Council of Trent. Found in Philip Schaff, The Creeds of Christendom (Grand Rapids: Baker, 1919), Canon IV, p. 119).
Penance is...necessary because we must expiate and make reparation for the punishment which is due our sins...We make satisfaction for our sins by every good act we perform in the state of grace but especially by prayer, penance and the practice of charity (John Hardon, The Question and Answer Catholic Catechism (Garden City: Image, 1981), Question #1320).
The Sacrifice of the altar... is no mere empty commemoration of the Passion and death of Jesus Christ, but a true and proper act of sacrifice. Christ, the eternal High Priest, in an unbloody way offers himself a most acceptable Victim to the eternal Father as He did upon the Cross...In the Mass, no less than on Calvary, Jesus really offers His life to His heavenly Father...The Mass, therefore, no less than the Cross, is expiatory for sins (emphasis mine) (John Hardon, The Question and Answer Catholic Catechism (Garden City: Image, 1981), Questions #1265, 1269, 1277).
If this is not enough to convince someone that Catholicism is a cult then I seriously question their understanding of the gospel message and salvation.