If you would believe that justification is by grace through faith PLUS the sacraments PLUS adherence to the RCC, then you would not be simply believing in Christ. And that's where the problem arises. There is always something ADDED to the finished work of Christ.
This argument would be sound if the presumptions in it about the Roman Catholic Church were true—but they are not!
Let’s take a look at the sacrament of water baptism. The Roman Catholic Church teaches that the grace for salvation is normatively conferred through the sacrament of water Baptism.
Rom. 5:12. Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned--
13. for until the Law sin was in the world, but sin is not imputed when there is no law.
14. Nevertheless death reigned from Adam until Moses, even over those who had not sinned in the likeness of the offense of Adam, who is a type of Him who was to come.
15. But the free gift is not like the transgression. For if by the transgression of the one the many died, much more did the grace of God and the gift by the grace of the one Man, Jesus Christ, abound to the many.
16. The gift is not like that which came through the one who sinned; for on the one hand the judgment arose from one transgression resulting in condemnation, but on the other hand the free gift arose from many transgressions resulting in justification.
17. For if by the transgression of the one, death reigned through the one, much more those who receive the abundance of grace and of the gift of righteousness will reign in life through the One, Jesus Christ.
18. So then as through one transgression there resulted condemnation to all men, even so through one act of righteousness there resulted justification of life to all men.
19. For as through the one man's disobedience the many were made sinners, even so through the obedience of the One the many will be made righteous.
20. The Law came in so that the transgression would increase; but where sin increased, grace abounded all the more,
21. so that, as sin reigned in death, even so grace would reign through righteousness to eternal life through Jesus Christ our Lord. (NASB, 1995)
According to this passage of Scripture, each one of us became accountable for Adam’s sin the moment that Adam ate the forbidden fruit. The proof of this, Paul argues, is that people died before the Law was given even though sin is not imputed when there is no law. Therefore, they could not have died for their own personal sins, but only for the sin that they committed in Adam. Therefore, every child born into this world is born with the guilt of their sin in Adam. This is, of course, why babies and infants who have not yet reached the age of reason are just as subject to death as are adults. And, of course, this is why all of the main-line Christian denominations baptize babies. Compare the First Epistle of Peter,
1 Pet. 3:18. For Christ also died for sins once for all, the just for the unjust, so that He might bring us to God, having been put to death in the flesh, but made alive in the spirit;
19. in which also He went and made proclamation to the spirits now in prison,
20. who once were disobedient, when the patience of God kept waiting in the days of Noah, during the construction of the ark, in which a few, that is, eight persons, were brought safely through the water.
21. Corresponding to that, baptism now saves you--not the removal of dirt from the flesh, but an appeal to God for a good conscience--through the resurrection of Jesus Christ,
22. who is at the right hand of God, having gone into heaven, after angels and authorities and powers had been subjected to Him. (NASB, 1995)
Notice especially the words in 1 Pet. 3:21, “baptism now saves you.” This is perhaps the most direct and clearest statement in all of the New Testament, that is, unless you happen to read a commentary on 1 Peter written by a Baptist who will probably believe differently. Peter believed that water baptism, when preceded by faith and repentance, was efficacious for salvation. Indeed, he is quoted by Luke in Acts 2:37-38 as saying that repentance and water baptism result in the forgiveness of one’s sins. The immediate context, indeed, the very next phrase, “and you will receive the gift of the Holy Spirit” conclusively proves that salvation is in view here. That is, in order to be saved, the believer in Christ,
according to Peter, must repent of his sins and be baptized in water.
Acts. 2:37. Now when they heard this, they were pierced to the heart, and said to Peter and the rest of the apostles, "Brethren, what shall we do?"
38. Peter said to them, "Repent, and each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the Holy Spirit. (NASB, 1995)
We see very clearly from these passages in the New Testament that infants are born with the guilt of the sin that they committed in Adam. We also very clearly see that, from Peter’s perspective, water baptism, when preceded by faith and repentance, is efficacious for salvation. This leaves us with the problem that infants are born with the guilt of the sin that they committed in Adam and yet they are not capable of faith and repentance. Therefore, we have the unresolved problem of whether or not baptizing infants is efficacious in remitting the sin that they committed in Adam.