The Fundamental Difference between Protestants and Roman Catholics is this: Rome does not see salvation from start to finish as the work of the triune God alone. Thus, justification before God (salvation) is not through faith alone by God's grace alone, but, as Rome teaches, salvation comes by faith + the meritorious works that a man must do (including unquestionable devotion to Rome and *religious* worship to Mary).

Rome denies that the work of Jesus Christ completely sufficient for salvation. That faith plus meritorious works must be employed for salvation (as Rome teaches) is, according to the the Apostle Paul, Christological heresy (cf. Gal. 1:6-8)--it rejects the work of Christ and hence rejects the Person of the Son, Jesus Christ. 

Rome's doctrine of Purgatory, for example, is that when a Christian dies without un-forgiven mortal sins, but who retain either un-forgiven daily sins (viz. venial sins) or "temporal punishment" due for sins are "purged" before entering heaven, so as to be made perfect. In other words, they must suffer for these sins in a place of torment (not hell, though) to be, so to speak, "scrubbed up" (viz. purified) before they can dwell with God in heaven.

Thus, the work of Christ, according to Rome, is not completely sufficient to atone and *justify* a sinner--for one must suffer for his or her own atonement  in order to become righteous (just) before God.

Though Scripture testifies in passages such as Hebrews 10:10-14

10:10: By his will we have been made holy through the offering of the body of Jesus Christ once for all.

10:11: And every priest stands day after day serving and offering the same sacrifices again and again – sacrifices that can never take away sins.

10:12: But when this priest had offered one sacrifice for sins for all time, he sat down at the right hand of God . . .

10:14: For by one offering he has perfected for all time those who are made holy.

And Romans 5:1-2 (see below):

Therefore, since we have been declared righteous by faith, we have peace with God through our Lord Jesus Christ, 5:2 through whom we have also obtained access by faith into this grace in which we stand, and we rejoice in the hope of God’s glory (NET; See Justification through Faith Alone ).

As with Mormons and Jehovah’s Witnesses, we must see Catholics as men and women in need of evangelism. One cannot biblically claim to be truly Christian and reject Christ as the sole means of salvation, justification through faith alone, and engage in creaturely worship.
 

ARTICLES

Sola Fide ("Faith Alone"): The Foundation of Protestantism: An Exegetical Look at Romans 4:4-8 by Edward Dalcour

 

A Concise look at Roman Catholicism and the worship of Mary by Edward Dalcour

The Empty Hand of Faith by James White
An "Electronic Tract" on the Nature of Saving Faith

Debate: Edward Dalcour vs. Roman Catholic Apologist Robert Sungenis: Romans 5:1: dikaiōthentes--"having been justified through faith"

"Therefore, having been justified by faith, we have peace with God through our Lord Jesus Christ."
 

Romans 5:1 expresses the doctrine of justification through faith alone (i.e., faith without additions or modifications) so clear that Roman Catholics must depart from normal exegesis to defend the crass works/salvation system that Rome has weighed down on her devotees. 

Debate: Edward Dalcour vs. Roman Catholic Mark Bonocore: The Early Church and Matthew 16:18: The Catholic Assertion of the "Rock" 

Matthew 16:18

"I also say to you that you are Peter, and upon this rock I will build My church; and the gates of Hades will not overpower it."

Matthew 16:18 has been used by the Catholic Church to promote the notion Peter was the first Pope and hence a papal succession. However, as Roman Catholic apologist, H. Burn-Murdock admits,

“None of the writings of the first two centuries describe St. Peter as a bishop of Rome” (The Development of the Papacy, 130f.).

In fact, no one before Callistus (A.D. 223) used Matt. 16:18 to support the primacy of the Roman *bishop* ("Pope" as termed in Catholicism)—no one.  Why? if in fact God ordained t that is to be the supreme and one and only true church earth: “there is no salvation outside the RCC” (extra Ecclesia, nullus salus):

“There is but one universal Church of the faithful, outside which no one at all is saved” (Pope Innocent III, Fourth Lateran Council, 1215)

“We declare, say, define, and pronounce that it is absolutely necessary for the salvation of every human creature to be subject to the Roman Pontiff” (Pope Boniface VIII, papal bull, Unam Sanctam, 1302).

The Canon is closed

In direct contrast to the Protestant concept of sola Scriptura ("Scripture alone") is sola Ecclesia (i.e., “Church alone”), which is clearly the marrow of groups such as Catholicism. Hence, it is not that the Catholic apologist does not have the ability to exegete, but he has no need, for the Church has done the job for him. For the Catholic sees his Church, not Scripture, as the final sole authority in all areas of life and theology (cf. Catechism of the Catholic Church, para. 84-85, 113).