“If the claims of the papacy cannot be proven from what we know of the historical Peter, there are, on the other hand, several undoubted facts in the real history of Peter which bear heavily upon those claims, namely: 1. That Peter was married, Matt. 8:14, took his wife with him on his missionary tours, 1 Cor. 9:5, and, according to a possible interpretation of the "coëlect" (sister), mentions her in 1 Pet. 5:13. Patristic tradition ascribes to him children, or at least a daughter (Petronilla). His wife is said to have suffered martyrdom in Rome before him. What right have the popes, in view of this example, to forbid clerical marriage? We pass by the equally striking contrast between the poverty of Peter, who had no silver nor gold (Acts 3:6) and the gorgeous display of the triple-crowned papacy in the middle ages and down to the recent collapse of the temporal power. 2. That in the Council at Jerusalem (Acts 15:1–11), Peter appears simply as the first speaker and debater, not as president and judge (James presided), and assumes no special prerogative, least of all an infallibility of judgment. According to the Vatican theory the whole question of circumcision ought to have been submitted to Peter rather than to a Council, and the decision ought to have gone out from him rather than from "the apostles and elders, brethren" (or "the elder brethren," 15:23). 3. That Peter was openly rebuked for inconsistency by a younger apostle at Antioch (Gal. 2:11–14). Peter’s conduct on that occasion is irreconcilable with his infallibility as to discipline; Paul’s conduct is irreconcilable with Peter’s alleged supremacy; and the whole scene, though perfectly plain, is so inconvenient to Roman and Romanizing views, that it has been variously distorted by patristic and Jesuit commentators, even into a theatrical farce gotten up by the apostles for the more effectual refutation of the Judaizers! 4. That, while the greatest of popes, from Leo I. down to Leo XIII. never cease to speak of their authority over all the bishops and all the churches, Peter, in his speeches in the Acts, never does so. And his Epistles, far from assuming any superiority over his "fellow-elders" and over "the clergy" (by which he means the Christian people), breathe the spirit of the sincerest humility and contain a prophetic warning against the besetting sins of the papacy, filthy avarice and lordly ambition (1 Pet. 5:1–3). Love of money and love of power are twin-sisters, and either of them is "a root of all evil." It is certainly very significant that the weaknesses even more than the virtues of the natural Peter—his boldness and presumption, his dread of the cross, his love for secular glory, his carnal zeal, his use of the sword, his sleepiness in Gethsemane—are faithfully reproduced in the history of the papacy; while the addresses and epistles of the converted and inspired Peter contain the most emphatic protest against the hierarchical pretensions and worldly vices of the papacy, and enjoin truly evangelical principles—the general priesthood and royalty of believers, apostolic poverty before the rich temple, obedience to God rather than man, yet with proper regard for the civil authorities, honorable marriage, condemnation of mental reservation in Ananias and Sapphira, and of simony in Simon Magus, liberal appreciation of heathen piety in Cornelius, opposition to the yoke of legal bondage, salvation in no other name but that of Jesus Christ.”
On his journey he stayed in England and met Edward Pusey and other Tractarians. His inaugural address on The Principle of Protestantism, delivered in German at Reading, Pennsylvania, in 1844, and published in German with an English version by John Williamson Nevin was a pioneer work in English in the field of symbolics (that is, the authoritative ecclesiastical formulations of religious doctrines in creeds or confessions). This address and the "Mercersburg Theology" which he taught seemed too pro-Catholic to some, and he was charged with heresy. But, at the synod at York in 1845, he was unanimously acquitted.
Schaff's broad views strongly influenced the German Reformed Church, through his teaching at Mercersburg, through his championship of English in German Reformed churches and schools in America, through his hymnal (1859), through his labours as chairman of the committee which prepared a new liturgy, and by his edition (1863) of the Heidelberg Catechism. His History of the Apostolic Church (in German, 1851; in English, 1853) and his History of the Christian Church (7 vols., 1858-1890), opened a new period in American study of ecclesiastical history.
In 1854, he visited Europe, representing the American German churches at the ecclesiastical diet at Frankfort and at the Swiss pastoral conference at Basel. He lectured in Germany on America, and received the degree of D.D. from Berlin.
In consequence of the ravages of the American Civil War the theological seminary at Mercersburg was closed for a while and so in 1863 Dr. Schaff became secretary of the Sabbath Committee (which fought the “continental Sunday”) in New York City, and held the position till 1870. In 1865 he founded the first German Sunday School in Stuttgart. In 1862-1867 he lectured on church history at Andover.
Schaff was a member of the Leipzig Historical Society, the Netherland Historical Society, and other historical and literary societies in Europe and America. He was one of the founders, and honorary secretary, of the American branch of the Evangelical Alliance, and was sent to Europe in 1869, 1872, and 1873 to arrange for the general conference of the Alliance, which, after two postponements on account of the Franco-Prussian War, was held in New York in October 1873. Schaff was also, in 1871, one of the Alliance delegates to the emperor of Russia to plead for the religious liberty of his subjects in the Baltic provinces.
He became a professor at Union Theological Seminary, New York City in 1870 holding first the chair of theological encyclopedia and Christian symbolism till 1873, of Hebrew and the cognate languages till 1874, of sacred literature till 1887, and finally of church history, till his death. He also served as president of the committee that translated the American Standard Version of the Bible, though he died before it was published in 1901.
His History of the Christian Church resembled Neander's work, though less biographical, and was pictorial rather than philosophical. He also wrote biographies, catechisms and hymnals for children, manuals of religious verse, lectures and essays on Dante, etc. He translated Johann Jakob Herzog's Real-Encyklopädie für protestantische Theologie und Kirche into English.