Does the Presbyterian Church hold to the teaching of "sola fide"? How is a person justified before God? Are there any major differences between Presbyterians and other non Roman Christians?
The first question is very easy: yes, the OPC believes in the solas of the Reformation, of which sola fide is one. When we say "only faith" we mean that none of man's works enter into the salvation equation except as fruit of the new life God imparts to those who believe in Jesus Christ.
The Westminster Confession of Faith states this in Chapter 11:
1. Those whom God effectually calleth, he also freely justifieth: not by infusing righteousness into them, but by pardoning their sins, and by accounting and accepting their persons as righteous; not for anything wrought in them, or done by them, but for Christ's sake alone; nor by imputing faith itself, the act of believing, or any other evangelical obedience to them, as their righteousness; but by imputing the obedience and satisfaction of Christ unto them, they receiving and resting on him and his righteousness, by faith; which faith they have not of themselves, it is the gift of God.
2. Faith, thus receiving and resting on Christ and his righteousness, is the alone instrument of justification: yet is it not alone in the person justified, but is ever accompanied with all other saving graces, and is no dead faith, but worketh by love.
3. Christ, by his obedience and death, did fully discharge the debt of all those that are thus justified, and did make a proper, real, and full satisfaction to his Father's justice in their behalf. Yet, inasmuch as he was given by the Father for them; and his obedience and satisfaction accepted in their stead; and both, freely, not for anything in them; their justification is only of free grace; that both the exact justice and rich grace of God might be glorified in the justification of sinners.
And the Westminster Shorter Catechism summarizes it this way:
Question 33, What is justification?
Justification is an act of God's free grace, wherein he pardoneth all our sins and accepted us as righteous in his sight, only for the righteousness of Christ imputed to us, and received by faith alone. [or sola fide!]
We are justified or declared righteous before God because we believe that God has done in Christ what we could never do in ourselves, pay for and remove all the guilt of our sins, and further, Christ gives and God imputes or reckons as being our very own, Christ's perfect righteousness to us so that we are only seen by God as though we had never sinned but as being perfectly obedient all for the sake and the merit of Jesus Christ. All of these blessings come to the person who believes what God has promised in the gospel, so faith is not a work but the instrument by which Christ's work becomes our own. God views any mixture of our own works with faith in Christ as another gospel. This is what Paul wrote against in the letter to the Galatians. We could say that the solas hang together: sola scriptura (only the Bible), sola christo (only Christ), sola fide (only faith), sola gratia (only grace), and sola deo gloria (only God is glorified) form a unity. A helpful book on this is The Case for Traditional Protestantism by Terry Johnson published by The Banner of Truth Trust.
Your other question about whether there are any differences between Presbyterians and other Protestants is best addressed first by recognizing that there are common truths of the Christian faith as seen in something like the Apostles Creed. Every person who says that they are a Christian should be able to sincerely and honestly affirm the things stated in that creed. Christianity is trinitarian, for instance, which excludes such groups as Unitarians or United Pentecostals (Jesus Only folks). Roman Catholics also should be able to affirm that the Bible teaches the doctrine of the Trinity. Likewise, the Apostles Creed affirms that Jesus Christ is the Son of God who truly came, lived, died, and rose on the third day and someone who claims to be a Christian should be able to say those things about Christ, but, alas, many liberal Protestants no longer believe in the divinity of Jesus Christ or in the reality of the resurrection. So here are two areas (among others) where within Protestantism you have lines drawn.
But Presbyterians hold to a fuller expression of biblical Christianity. While Presbyterians affirm the Apostles Creed along with Lutherans, Baptists, etc., they also confess the truths stated in the Reformed confessions like the Westminster Confession of Faith and Catechisms (click here to read them for yourself) or the Heidelberg Catechism, Belgic Confession or Canons of Dordt. Specifically the Reformed creeds develop and unfold many more biblical truths than we find in the Apostles or Nicene Creed on a variety of biblical doctrines. Most notably Presbyterians would confess that God alone saves, that man is dead in sin and unable to save himself or to choose Christ until God acts upon the soul to give spiritual life (what Paul describes in Ephesians 1-2 or Romans 9-11). This affirmation of God's sovereign power in salvation sets Presbyterians off from those who believe that man has in himself the ability to choose, those people called Arminians, who are found in Methodist churches or many Baptists. So while Presbyterians could stand with a Catholic or a Methodist in confessing that God is three persons in one Godhead and that Jesus is the only begotten, eternal Son of God, God and man in one person, Presbyterians would say that there are other truths where they will have to part company because they confess, unlike Roman Catholics, that faith is the alone instrument of salvation or unlike Methodists, man cannot respond to God until God first works in man.
I hope this is enough to get you thinking and reading.
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