This is from “The Doctrine and History of Worship in the Reformed Presbyterian Church of North America,” from a thesis submitted in May 1982 for an STM at Concordia Seminary by John Allen Delivuk. This is the section “Reformed Presbyterians in Hymn-Singing Churches”, pp. 142-143.
Since Reformed Presbyterians believe that it is wrong to sing hymns and use instruments in worship, what do they do when worshipping in denominations where hymns and/or organs are used? This question was asked by Dr. J. G. Vos, a well-known editor and theologian among Reformed Presbyterians. Dr. Vos answered, “My practice is to remain silent under such circumstances.” To the credit of other Christians, Dr. Vos continues, “I often attend such churches, and find that where my convictions are known, they are usually respected by others.” This would reflect the normal practice among Covenanters.
There is also the question of what ministers should do when leading worship in other denominations. In 1886 Synod recommended the following policy for ministers preaching in hymn-singing churches:
- That in all cases our ministers should use our own version of the Psalms, if possible, but use any good version when it is necessary.
- That in accordance with our received principle, that worship must be of divine appointment, our ministers cannot consistently give out hymns of human composition in religious worship.
- That ministers may preach the gospel where hymns and instrumental music are used, provided it is understood that they do not sanction the use of them, but they may not conduct the services or worship unless allowed to use the Scripture Psalms.
The common practice of recent times when preaching in churches that use hymns and instruments is for the Reformed Presbyterian minister to ask one of the elders or deacons (depending on polity) to preside. This avoids the problem of seeming to approve hymns by announcing them. While the hymns are sung, the Reformed Presbyterian minister will remain silent.
 J. G. Vos, “Blue Banner Question Box,” Blue Banner Faith and Life 22 (April-June 1967):115.
 Minutes, 1886, p. 271.