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.