The church I attend, we use for worshipping two books; The Book of Psalter For Singing and the Trinity Hymnal. The latter contains hymns that were written by several Church Fathers. For example;

1.) Ambrose of Milan, who had a great influence upon Augustine, wrote hymns for the Church when he lived (340-397). One of those hymns Luther translated in 1524 for the German Church.

2.) Gregory the Great (509-604).

3.) Gregory Nazianzen (325-390).

4.) Clemens (348-413).

5.) Clement of Alexandria ( lived around AD200).

Etc.

Can someone help me with the below questions that I have as a result from the above information;

Were these Church Fathers in error in writing these hymns by going against RWP according to EP's?

It seems that EP's say that the church has always sang psalms, do they mean in addition to hymns or that the Church of the OT always sang psalms?

If the church in the OT always sang psalms, why do Jews sing other songs that are not psalms?