Do you have any information on specifically what ways the Orthodox Presbyterian Church (OPC) differs with the Presbyterian Church in the United States of America (PCUSA) and the Evangelical Presbyterian Church (EPC)?
This is a most timely question. To begin my answer, here is a quotation from a helpful online publication at the OPC website (http://www.opc.org), entitled "What is the Orthodox Presbyterian Church?" (http://opc.org/whatis.html#I). You may want to read it in its entirety.
"During the nineteenth century, the Presbyterian Church in the U.S.A. was largely a strong and faithful church. But liberalism began to creep in from Europe, and little was done to check its spread. In 1924 about 1,300 (out of 10,000) Presbyterian ministers signed the liberal Auburn Affirmation, which denied that the Bible was without error and declared that belief in such essential doctrines as Christ's substitutionary atonement and his bodily resurrection should not be made "tests for ordination or for good standing in our church." Unbelief was taking over the church.
"Princeton Theological Seminary in Princeton, New Jersey, remained a bastion of Presbyterian orthodoxy. But in 1929 its Board was reorganized with a mandate to put liberal professors on the faculty. Four Princeton professors resigned and (with the support of others) established Westminster Theological Seminary in Philadelphia as an independent institution to continue teaching biblical Christianity.
"The leading opponent of liberalism in those days was J. Gresham Machen, a Presbyterian minister and professor at Princeton (and later Westminster). When he exposed the modernist unbelief that permeated the foreign missions of the Presbyterian Church in the U.S.A., the General Assembly in 1933 refused to do anything about it. Because he and others would only support missionaries who were actually preaching the gospel, they established the Independent Board for Presbyterian Foreign Missions. The 1934 Assembly condemned their action, and they were soon deposed from office. In response, 34 ministers, 17 ruling elders, and 79 laymen met in Philadelphia on June 11, 1936, to constitute the Presbyterian Church of America. (Because of a lawsuit brought by the Presbyterian Church in the U.S.A., the name of the new church was changed to the Orthodox Presbyterian Church in 1939.) They wanted to "continue the true spiritual succession of the Presbyterian Church in the U.S.A." They hoped that a mass exodus of Bible-believing Christians would swell the ranks of the new denomination, but it never happened. Then, on January 1, 1937, Machen's untimely death dealt a severe blow to the new church."
This in itself is a radical difference from the PCUSA where the Westminster Confession and catechisms are more looked upon as historical documents that summarize what the church USED TO BELIEVE. Ministers and elders in the PCUSA are not required to subscribe to those confessional statements. In fact, I myself experienced first hand in 1979-1981 in what was then the UPCUSA, the courts of the church upholding a minister who openly denied the deity of Christ and who would not affirm either the bodily resurrection of Christ or the blood atonement. That is what led me to join the OPC shortly thereafter. The PCUSA has condoned denials of the Biblical faith and disciplined those men like Dr. Machen and others who have sought to hold her accountable to the Word of God.
In the OPC we believe that the marks of the true church are the faithful preaching of the Word of God, the faithful administration of the sacraments of baptism and the Lord's Supper, and the faithful administration of church discipline. In all things the Scriptures are the rule of faith and practice. Those who are delinquent in doctrine or life are subject to the discipline of the church.
While the Evangelical Presbyterian Church generally has a more biblically conservative bent than the PCUSA, the OPC differs from the Evangelical Presbyterian Church particularly in two matters.
First, we wholeheartedly affirm the teaching of the apostle Paul which forbids women teaching or having authority over men. Thus, we do not ordain women to the offices of minister, ruling elder, or deacon. The EPC does permit the ordination of women.
Second, the OPC is also united in its belief that the charismatic gifts such as prophecy and the speaking of tongues ceased with the end of the apostolic age. The EPC believes that those gifts continue into the present day. The OPC is persuaded that those gifts were specifically associated with the apostolic era while the Scriptures were still being written as signs of the authenticity of the apostles. Once the canon of Scripture was completed, those gifts ceased and the Scriptures alone are the sole, sufficient authority given by God for the instruction of His church. We confess, "The whole counsel of God concerning all things necessary for his own glory, man's salvation, faith and life, is either expressly set down in Scripture, or by good and necessary consequence may be deduced from Scripture: unto which nothing at any time is to be added, whether by new revelations of the Spirit, or traditions of men" (WCF I.6). And further we confess, "The supreme judge by which all controversies of religion are to be determined, and all decrees of councils, opinions of ancient writers, doctrines of men, and private spirits, are to be examined, and in whose sentence we are to rest, can be no other but the Holy Spirit speaking in the Scripture" (WCF I.10).
While more could be said, I hope this is helpful.
May the Lord bless your continued pursuit of His truth.
R. Daniel Knox, Pastor
Grace Orthodox Presbyterian Church