"Hymns of human composition are used so commonly now in public worship by Presbyterian churches that it is difficult to believe that the practice is not a hundred years old, and that in some of the churches it is of very recent date. On the supposition that it is good and dutiful and wise to sing such hymns in worship, it is equally difficult to account for the neglect of the churches at the time of the Reformation, and for generations afterwards. What could have so blinded the reformers as to make them reject hymns and sing the Psalms alone? How could the Westminster Divines, in framing their Confession of Faith and Directory for Worship, have been so unanimous in the blunder that the service of praise is to consist of the 'singing of Psalms?'And apart from the aspect of duty, how could the Presbyterian churches, for about a hundred and fifty or two hundred years after the Westminster Assembly, have been so insensible to the power of hymns as an attractive addition to their public services? We cannot by any means understand how it was that, if it was dutiful to use hymns in worship, the reformers did not discover the Scriptural warrant for the duty, especially as hymns had been used for centuries by the Church of Rome. Nor can we understand how they rejected the hymns and used the Psalms alone, unless on the supposition that they believed the use of hymns to be part of the will-worship of Rome. If they were wrong on this point, then Rome and our modern Presbyterian churches are right. In that case, the Puritans and Covenanters were fanatics, and Romanists were truly enlightened! And most of our Presbyterian churches of the present day were fanatical too, and did not become truly enlightened and liberal till they got back to the Romish practice!"
Related information about Exclusive Psalmody and the Westminster Assembly from a email (emphases added) by Greg Price follows:
If I can be of help in your contending for the faith, let me know. That citation in my sermon to which you refer was an official order issued by the House of Commons (upon recommendation from the Westminster Assembly).
The Westminster Assembly sent to the House of Lords (Nov.14,1645) the following message concerning the Psalter of Rouse:
"The Assembly of Divines having received from this Honourable House an order, bearing date October 7,1645, to read over and judge of two Books of David's Psalms, composed in English metre, by Mr. William Barton, and thereupon to return their judgment to this Honorable House, do humbly certify, That they had long before received an order from the Honorable House of Commons, bearing date November 20,1643, to give their judgment touching the Psalms composed in metre by Mr. Rouse, a Member of that House; and that thereupon there was a Committee appointed by this Assembly to consider of these Psalms; and that the same Committee had with much care perused, and with great diligence concurred with the same Learned Gentleman, to amend and perfect his copy, and had fully finished the Work, before they received the said order from the Honorable House of Lords; and withal that the greatest part of this version was sent to the General Assembly of the Church of Scotland, and there put into the hands of a Committee, and by that Committee, so far as they have examined it, very well approved; yet, in obedience to the order of this Honorable House, they appointed a Committee to consider thereof; and, upon the whole matter, do find reason to certify this Honorable House, That albeit the said Mr. Barton hath taken very good and commendable pains in his Metphrase, yet the other version, so exactly perused and amended by the said Mr. Rouse and the Committee of the Assembly with long and great labor, is so closely framed according to the Original Text, as that we humbly conceive it will be useful for the edification of the Church" (cited from The Letters and Journals of RobertBaillie).
The House of Commons also gave their wholehearted endorsement of Rouse's Psalter in the following words (April 15,1646):
"Ordered, That the Book of Psalms, set forth by Mr. Rous, and perused by the Assembly of Divines, be forthwith printed in sundry volumes: And that the said Psalms, and none other, shall, after the first of January next, be sung in all Churches and Chapels within the Kingdom of England, Dominion of Wales, and Town of Berwick-upon-Tweede; and that it be referred to Mr. Rous, to take care for the true printing thereof" (Cited from The Letters and Journals of Robert Baillie, 3:539).
Furthermore, the Church of Scotland took the following action (Nov.23,1649):
"The Commission of the General Assembly having with great diligence considered the Paraphrase of the Psalms in Meter, sent from the Assembly of Divines in England by our Commissioners, whilst they were there, as it is corrected by former General Assemblies, Committees from them, and now at last by the Brethren deputed by the late Assembly for that purpose: And having exactly examined the same, do approve the said Paraphrase, as it is now compiled; And therefore, according to the power given them by the said Assembly, do appoint it to be printed and published for public use: Hereby authorizing the same to be the only Paraphrase, of the Psalms of David to be sung in the Kirk of Scotland: and discharging the old Paraphrase and any other than this new Paraphrase, to be made use of in any congregation or family after the first day of May in the year 1650; And for Uniformity in this part of the Worship of God, do seriously recommend to Presbyteries to cause make public intimation of this Act, and take special care that the same be tymeously [i.e. timely--GLP] put to execution, and duly observed" (Cited from The Letters and Journals of Robert Baillie, 3:548).
Finally, the Committee of Estates (i.e. the Committee for the Scottish Parliament) issued the following order (Jan.8,1650):
"The Committee of Estates having considered the English Paraphrase of the Psalms of David in Meter, presented this day unto them by the Commission of the General Assembly, together with their Act and the Act of the late Assembly, approving the said Paraphrase, and appointing the same to be sung through this Kirk. Therefore, the Committee doth also approve the said Paraphrase, and interpone their authority for the publishing and practicing thereof; hereby ordaining the same, and no other to be made use of throughout this Kingdom, according to the tenor of the said Acts of the General Assembly and their Commissioners" (Cited from The Letters and Journals of Robert Baillie, 3:548,549).