There appears some confusion about who was, or what counts as, the first pastor of Jerusalem Chapel. Ystalyfera. The South Wales Voice of 7th July 1951 states:-

THE FIRST PASTOR
For many years however, they could not afford the services of a pastor; the first to Officiate in that capacity was the Rev Ivor Jenkins, who retired in 1913

Conversely, the South Wales Voice for 13th December 1957 states:-

The first Minister was the late Rev John Jeffries who took office in 1884 and was succeeded by the Rev J Ivor Jenkins in 1902 and he in turn was followed in 1914 by the Rev D W Stephens.

The South Wales Voice of 22nd October 1932 records the death of John Jeffreys [sic]:-

DEATH OF FORMER JERUSALEM PASTOR
The death has taken place of the Rev John Jeffreys Cwmbach Carmarthenshire, a former Pastor of Jerusalem Chapel. The Rev gentleman was in charge of Jerusalem from 1882 to 1914.
The funeral is on Thursday and Messrs R W Hopkin, E R Morgan and David Davies were appointed to represent the church at the funeral.
The Rev W Peregrine Jones was present on behalf of Bethania Chapel Lower Cwmtwrch.