We must distinguish between Catholicism and Romanism. The former embraces the ancient Oriental church, the mediaeval church, and we may say, in a wider sense, all the modern evangelical churches. Romanism is the Latin church turned against the Reformation, consolidated by the Council of Trent and completed by the Vatican Council of 1870 with its dogma of papal absolutism and papal infallibility. Mediaeval Catholicism is pre-evangelical, looking to the Reformation; modern Romanism is anti-evangelical, condemning the Reformation, yet holding with unyielding tenacity the oecumenical doctrines once sanctioned, and doing this all the more by virtue of its claim to infallibility.
The distinction between pre-Reformation Catholicism and post-Reformation Romanism, in their attitude towards Protestantism, has its historical antecedent and parallel in the distinction between pre-Christian Israel which prepared the way for Christianity, and post-Christian Judaism which opposed it as an apostasy.
Catholicism and Protestantism represent two distinct types of Christianity which sprang from the same root, but differ in the branches.
Catholicism is legal Christianity which served to the barbarian nations of the Middle Ages as a necessary school of discipline; Protestantism is evangelical Christianity which answers the age of independent manhood. Catholicism is traditional, hierarchical, ritualistic, conservative; Protestantism is biblical, democratic, spiritual, progressive. The former is ruled by the principle of authority, the latter by the principle of freedom. But the law, by awakening a sense of sin and exciting a desire for redemption, leads to the gospel; parental authority is a school of freedom; filial obedience looks to manly self-government.
The characteristic features of mediaeval Catholicism are intensified by Romanism, yet without destroying the underlying unity.
Romanism and orthodox Protestantism believe in one God, Father, Son, and Holy Spirit, and in one divine-human Lord and Saviour of the race. They accept in common the Holy Scriptures and the oecumenical faith. They agree in every article of the Apostles’ Creed. What unites them is far deeper, stronger and more important than what divides them.
But Romanism holds also a large number of "traditions of the elders," which Protestantism rejects as extra-scriptural or anti-scriptural; such are the papacy, the worship of saints and relics, transubstantiation, the sacrifice of the mass, prayers and masses for the dead, works of supererogation, purgatory, indulgences, the system of monasticism with its perpetual vows and ascetic practices, besides many superstitious rites and ceremonies.
Protestantism, on the other hand, revived and developed the Augustinian doctrines of sin and grace; it proclaimed the sovereignty of divine mercy in man’s salvation, the sufficiency of the Scriptures as a rule of faith, and the sufficiency of Christ’s merit as a source of justification; it asserted the right of direct access to the Word of God and the throne of grace, without human mediators; it secured Christian freedom from bondage; it substituted social morality for monkish asceticism, and a simple, spiritual worship for an imposing ceremonialism that addresses the senses and imagination rather than the intellect and the heart.
The difference between the Catholic and Protestant churches was typically foreshadowed by the difference between Jewish and Gentile Christianity in the apostolic age, which anticipated, as it were, the whole future course of church history. The question of circumcision or the keeping of the Mosaic law, as a condition of church membership, threatened a split at the Council of Jerusalem, but was solved by the wisdom and charity of the apostles, who agreed that Jews and Gentiles alike are "saved through the grace of the Lord Jesus" (Acts 15:11). Yet even after the settlement of the controversy by the Jerusalem compromise Paul got into a sharp conflict with Peter at Antioch on the same question, and protested against his older colleague for denying by his timid conduct his better conviction, and disowning the Gentile brethren. It is not accidental that the Roman Church professes to be built on Peter and regards him as the first pope; while the Reformers appealed chiefly to Paul and found in his epistles to the Galatians and Romans the bulwark of their anthropology and soteriology, and their doctrine of Christian freedom. The collision between Paul and Peter was only temporary; and so the war between Protestantism and Romanism will ultimately pass away in God’s own good time.