The Second London Confession cribbed whole sections of the Westminster Confession; many Particular Baptists had been using the Westminster Confession and — after the Act of Toleration made assembly practical again — decided to endorse it, changing it to emphasize specifically Baptist beliefs.
The similarities — the Particular Baptists and the Presbyterians both shared a Calvinist theology — are to be expected, but the differences are also worth noting.
The Baptists excised a section on Christian Liberty, rejecting language that granted civil authority power to punish religious misdemeanors:
"And because the powers which God has ordained, and the liberty which Christ has purchased are not intended by God to destroy, but mutually to uphold and preserve one another, they who, upon pretence of Christian liberty, shall oppose any lawful power, or the lawful exercise of it, whether it be civil or ecclesiastical, resist the ordinance of God. And, for their publishing of such opinions, or maintaining of such practices, as are contrary to the light of nature, or to the known principles of Christianity (whether concerning faith, worship, or conversation), or to the power of godliness; or, such erroneous opinions or practices, as either in their own nature, or in the manner of publishing or maintaining them, are destructive to the external peace and order which Christ has established in the Church, they may lawfully be called to account, and proceeded against, by the censures of the Church. and by the power of the civil magistrate."
In similar fashion, the London Confession also eliminates a section on Civil Magistrates that declares "he has 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, administrated, and observed."
The London Confession discards all the sacramental language connected with baptism and the Lord's Supper and makes suitable changes to sections on the ordinance to reflect Baptist belief, such as immersion and rejection of infant baptism.
They also rewrote the section on the Church to add Baptist polity, insisting that "To each of these Churches thus gathered, according to his mind, declared in his word, he hath given all that power and authority, which is any way needfull, for their carrying on that order in worship, and discipline, which he hath instituted for them to observe; with commands, and rules, for the due and right exerting, and executing of that power."
The Baptists thus threw out the section on Synods and Councils, rejecting not only the authority of such bodies but of the civil magistrate's right to convoke them. (The Baptists also trimmed the section on marriage and divorce, whacking any mentioned of divorce.)
Thus, while it is similar in many areas to the Westminster, it also reflects a distinctly Baptist faith and practice.
(The Second London Confession is identical to the Philadelphia Confession of 1742, with the exception that the latter added singing and laying on of hands as ordinances.)