WCF 20.1 provides the basis for Christian liberty. This has been purchased for us by Christ under the gospel. This pertains not only to freedom from sin and its consequences but also to the liberty won by Christ that brings "deliverance from bondage to man... He alone is Lord of our conscience. We are thus freed from anything that is contrary to his Word in matters of faith and worship, we are also freed from the obligation to follow commands that are additional to what he has revealed in his Word. In the context of the Laudian repression, this was a powerfully liberating statement. Indeed, Christians are prohibited from yielding their consciences to the whims of man.... Samuel Rutherford summed it up pithily in his comment: 'It is in our power to vow, but not in the church's power to command us to vow'. (pp. 300-301)The imposition of liturgical demands by the Church/State authority in England had moved many of the best clergy in England to more fully embrace the regulative principle of worship as found in embryonic form in the Thirty-Nine Articles of Religion. A case can certainly be made that the RPW grows out of Article VI:
Holy Scriptures containeth all things necessary to salvation: so that whatsoever is not read therein, nor may be proved thereby, is not to be required of any man, that it should be believed as an article of the faith, or be thought requisite or necessary to salvation.Yet some will certainly argue that even under the reformer Thomas Cranmer the Book of Common Prayer was imposed by force of law. One need only look to the ordination of Bishop John Hooper as an example. But rather than a proof for the imposition of the BCP, this is an example of the Erasmain mindset the early reformers were born into, a hangover from the years of Christendom under the Holy Roman Empire. The reforming of doctrine always precedes the reforming of practice, which comes about more slowly.
Robert Letham continues, explaining why the Westminster Divines thus rejected the set liturgy of the Church of England:
When we reflect on the drastic imposition of the Book of Common Prayer by the Elizabethan settlement and its aftermath [the Laudian repressions], we see why the Assembly produced a directory of worship giving freedom to individual ministers to conduct worship services within the boundaries of the regulative principle of Scripture. It was the binding legal requirement, imposed by the crown, with penalties attached, that was the real nub of the problem with the liturgy for Puritan minds. While opposing the legal imposition of set liturgies, the Assembly was not abandoning liturgies as such. The Directory for Publick Worship of God contains a range of model prayers to be used in the regular service, at the start, before the sermon, after the sermon, before and after baptism, during and after communion, at the solemnization of marriage, in visiting the sick, and at public solemn fasting. Even John Owen, a few years too young to have been appointed to the Assembly, when writing on liturgies, stressed that he was not opposed to them or to the Book of Common Prayer. but to their imposition by law, with the forbidding of the slightest deviation from the set words. The standard practice of the Reformed churches had been to have a liturgy with set prayers; the problem for the divines was the rigid impositions and the repressive, punitive [state] sanctions for failure to comply. (pp.303-304)In A Discourse on Liturgies and Their Imposition John Owen elaborates concerning his objections to the BCP liturgy and its use:
They who are willing to take it upon their consciences that the best way to serve God in the church, or the best ability that they have for the discharge of their duty therein, consists in the reading of such a book (for I suppose they will grant that they ought to serve God with the best they have), shall not by me be opposed in their way and practice. It is only about its imposition, and the necessity of its observance by virtue of that imposition, that we discourse. Now, the present command is, that such a liturgy be always used in the public worship of God, and that without the use or reading of it the ordinances of the gospel be not administered at any time, nor in any place, with strong pleas for the obligation arising from that command, making the omission of its observance to be sinful. (chapter 7)John Owen highlights that the main objection to the use of the BCP was its imposition by force of law. It was this imposition, repressive due to the penalties attached and its lack of Scriptural warrant, that ran afoul of the Christians's liberty of conscience in Christ. And particularly onerous, regarding these laws under Archbishop Laud, was the restriction limiting sermons and thus the preaching of the gospel. Robert Letham adds more insight to Owen's thinking in a footnote found on page 304:
25. John Owen, "A Discourse Concerning Liturgies, and Their Imposition" (1662), in Works of John Owen, 15:33, where he states, " I do not in especial intend the liturgy now in use in England, any further than to make it an instance of such imposed liturgies, whereof we treat." He adds, "Nor, secondly, do I oppose the directive part of this liturgy as to the reading of Scripture... nor the composition of forms of prayer suitable to the nature of the institutions to which they relate, so they be not imposed on the administrators of them to be read precisely as prescribed. But, thirdly, this is that alone which I shall speak unto,--the composing of forms of prayer in the worship of God... to be used by the ministers of the churches, in all public assemblies, by a precise reading of the words prescribed unto them, with commands for the reading of other things, which they are not to omit, upon the penalty contained in the sanction of the whole service and the several parts of it." The problem for Owen and his friends, he explains, was that this imposition was accompanied by a restriction on preaching. Later he refers to "the prescription of the liturgy, to be used as prescribed: (15:47), and to "the precise reading and pronouncing of the words set down therein, without alteration, diminution, or addition" (15:49). Kelly is wrong when he writes that Owen was "against all set liturgies" ("Puritan Regulative Principle," 2:74)