Once Exclusively Sung

John H. Gerstner, Douglas F. Kelly and Philip Rollinson, A Guide to the Westminster Confession of Faith: Commentary, pp. 104-105:

The singing of Psalms has also now just about died out. Originally Reformed churches only allowed congregational singing of the Psalms, because they were authentically and authoritatively Biblical and hence lacked the potential contamination of false teaching which might come from the mere human authorship of hymns. Subsequently, however, hymns (which are not even mentioned by the Confession) have been introduced into Reformed liturgical use and have virtually driven out what is mentioned and was once exclusively sung.

The Genevan Psalter and Common Grace

John H. Gerstner, "Singing the Words God Has Put in Our Mouths" (originally published in The Hymn, January 1953), in John H. Gerstner: The Early Writings, pp. 204-205:

The third theological foundation of the Genevan Psalter was the doctrine of common grace. By this it was recognized that there are two types of divine gifts -- supernatural and natural. The former are the virtues wrought in the soul by a special work of grace; the latter are those which pertain to secular matters and are distributed to all, not to saints only; as a matter of fact, often to sinners. But, wherever they were, Calvin recognized these and used them for his purposes. Skill in music is a natural rather than a supernatural skill, but Calvin was ever on the alert to capture this for the worship of God. Thus, at his Academy in Geneva, he made music required four hours each week. The choir thereby trained in this skill was to lead the people so they could, under its leadership, cultivate the same skill. Acting on this same principle, Calvin was quick to appreciate the able -- though not excessively orthodox -- Marot, and to stand by the gifted composer, Bourgeois, who was thrown into prison for breaking some of the rigid disciplines of Geneva of which Calvin was himself the main author. Abraham Kuyper, in his admirable Lectures on Calvinism, so aptly remarks, "Music...would flourish, henceforth, not within the narrow limitation of particular grace, but in the wide and fertile fields of common grace."4

4 Lectures on Calvinism, p. 228.