The Westminster Standards (including the Confession of Faith and Catechisms) were the fruit of the covenanted uniformity aimed at in the Solemn League and Covenant. The relationship between the Westminster Standards and the Solemn League and Covenant is so close, in fact, that to truly adhere to the Standards requires that an individual or church also adhere to the Solemn League and Covenant. In other words, all true Presbyterians must also be Covenanters. This is clear from 'The Directory for the Ordination of Ministers' in the original (1648) Westminster Standards' 'Form of Presbyterial Church-Government' where it says that every candidate for the ministry must 'bring with him a testimonial of his taking the Covenant of the three kingdoms,' i.e., the Solemn League and Covenant. All faithful Presbyterian ministers must adhere to the Covenant. As well, the General Assembly of the Church of Scotland ruled in 1648 'that all young students take the covenant at their first entry to colleges; and that hereafter all persons whosoever take the covenant at their first receiving the sacrament of the Lord's supper.' In other words, people who would not take the Covenant could not partake of the Lord's supper." - Dr. Michael Wagner (on the Puritan Hard Drive)
There was one great, and even sublime idea, brought somewhat indefinitely before the Westminster Assembly, which has not yet been realized, the idea of a Protestant union throughout Christendom, not merely for the purpose of counterbalancing Popery, but in order to purify, strengthen, and unite all true Christian churches, so that with combined energy and zeal they might go forth, in glad compliance with the Redeemer's commands, teaching all nations, and preaching the everlasting gospel to every creature under heaven. This truly magnificent, and also truly Christian idea, seems to have originated in the mind of that distinguished man, Alexander Henderson. It was suggested by him to the Scottish commissioners, and by them partially brought before the English Parliament, requesting them to direct the Assembly to write letters to the Protestant Churches in France, Holland, Switzerland, and other Reformed Churches. ... and along with these letters were sent copies of the Solemn League and Covenant, a document which might itself form the basis of such a Protestant union. - William Hetherington, History of the Westminster Assemblyon the Puritan Hard Drive.