American Presbyterians have indeed made a number of significant changes to the Westminster Confession through the years. Some portions of the Confession have been revised (or outright rejected) by most American Presbyterians, while other changes provoked (or were the result of) debate and division. The most significant changes can be broken down as follows:

• On the civil magistrate (1788–1799)
• Semi-Arminianism in the Cumberland Presbyterian Church (1829)
• Expansion and modernization (1887–1925)
• Conservatism in the Orthodox Presbyterian Church (1936)

Civil magistrates

The earliest major changes, and those affecting perhaps the most American Presbyterians, are related to the Westminster Confession's treatment of the authority and powers of the civil magistrate. The section at the root of the issue was 23.3, beginning with "yet he hath authority":

The civil magistrate may not assume to himself the administration of the Word and sacraments, or the power of the keys of the kingdom of heaven: yet he hath authority, and it is his duty, to take order, that unity and peace be preserved in the Church, that the truth of God be kept pure and entire; that all blasphemies and heresies be suppressed; all corruptions and abuses in worship and discipline prevented or reformed; and all the ordinances of God duly settled, administered, and observed. For the better effecting whereof, he hath power to call synods, to be present at them, and to provide that whatsoever is transacted in them be according to the mind of God.

18th century American Presbyterians objected to this language, especially the ideas that the civil magistrate could suppress heresy and control synods. As a result, the Presbyterian Church in the USA (PCUSA) approved notable changes in 1789:¹

• 20.4 – removal of "and by the power of the civil magistrate" regarding means of being called to account for sin
• 23.3 – near total rewrite, making the magistrate a "nursing father" rather than an authority over the church
• 31.2 – rewrite and consolidation into 31.1, for consistency with 23.3 and the magistrate's lack of power to call synods

The Associate Reformed Church modified the same sections in 1799, using different language but achieving similar results.²

The Cumberland Presbyterian Church

In the early 19th century, the Cumberland Presbyterian Church split from the PCUSA over issues similar to those of the Old Side–New Side controversy; it wanted to relax educational and doctrinal requirements for its pasters as they sought to minister in the frontier alongside the successful Methodists and Baptists. They established a denomination in 1829, and adopted a Confession based on the 1789 American revision of Westminster but with a number of Arminian tendencies:³

• Rewrite of Chapter 3, originally "Of God's Eternal Decree," including the complete ommission of sections 3–8. The associated note says that Calvinists drive "rational, accountable man into the asylum of fate."
• Modifications to chapters 9, 10, and 11 remove the doctrine of unconditional election, allow the possibility of entire sanctification, and indicate that all infants that die, not merely elect infants, are saved.

Expansion and modernization

The next round of significant modifications occurred in 1903, by the PCUSA. A few years prior, in 1887, it had excised the last phrase of section 24.4, prohibiting marriage to a close relative of a deceased spouse. But the changes of 1903 were more drastic:⁴

• 16.7 – softening of language regarding the sinfulness of the unsaved
• 22.3 – removal of the sentence declaring it sinful to refuse to make a just oath when required by lawful authority
• 25.6 – rewrite to remove explicit mentions of the Pope and Antichrist
• 34 – new chapter entitled, "Of the Holy Spirit," describing the role and work of the Holy Spirit
• 35 – new chapter entitled, "Of the Love of God and Missions," emphasizing God's desire that all be saved and the importance of missions

As an aside, not all bodies desiring these kinds of changes modified the WCF. For example, the United Presbyterian Church of North America adopted a Confessional Statement in 1925 that was meant to supersede the original Westminster Standards where the two differed.⁵

Orthodox Presbyterian Church

The Orthodox Presbyterian Church split from the PCUSA in the midst of the Fundamentalist–Modernist Controversy; its leaders opposed the theological liberalization of the PCUSA. As such, it rejected most of the changes made by its parent denomination in 1903, but accepted those made prior to that. Its significant differences from the 1647 edition can be summarized as follows:

• 20.4, 23.3, and 31.2 – retain the 1788 American revisions regarding the civil magistrate
• 24.4 – retain the 1877 revision regarding remarriage
• 22.3 – accept the 1903 revision regarding refusing oaths
• 25.6 – excise the clause identifying the Pope as the Antichrist (not rewriting the entire section, as in the 1903 revision)

The OPC recognizes that these revisions are "not insignificant," but calculates that they only affect 145 of the 12063 words in the Confession, or 1.2%.⁶

Summary

It's certainly true that American Presbyterians have significantly modified the Westminster Confession; some much more so than others. Over the years some denominations have seen fit to change even its language on major Reformed doctrines, whether to jettison them or soften them. But, for what it is worth, the version used by the OPC (and, by extension, the PCA) has been less extensively modified.

Bibliography

For analysis of editions of the Westminster Confession prior to 1900, see B. B. Warfield, The Printing of the Westminster Confession, II. He provides a listing of all changes, including minor ones, between the original 1647 and PCUSA 1896 editions on page 116. His treatment of groups outside the PCUSA is less extensive; for example, he does not mention the Cumberland Presbyterian Church.

For a helpful treatment of the American revisions of 1788, see Lee Irons, "The 1788 American Revision of the Westminster Standards." For analysis, see Hart and Muether, "Turning Points in American Presbyterians History, Part 4"

For more general treatment of major Protestant confessions through the early 20th century, see Philip Schaff's Creeds of Christendom, Volume III.

References

1. Irons.
2. Constitution and Standards, pages 94, 111, and 134.
3. Schaff, 3.3.2
4. Schaff, 4.1.3
5. Schaff, 4.1.5
6. OPC, "American Revisions to the Westminster Confession of Faith"