Hi
Checked at Caernarfon Archives, no records available for Salem. According to a book re Baptists by J Spinther James MA, published 1903, they started meeting around 1783/85, first at a farmhouse called Dyffryn. Tenant was, 'un oedd ddim yn proffesu' (not sure whether author means 'one who was not religious', or 'one that was not a supporter of the Baptist cause'). Tenant was threatened with eviction if he continued to allow them to meet there, so William Williams, Hirdre allowed them to meet at Ty'n Donnen. When that became too small he, Wm Wms, gave them a 99 year lease for 10 shillings a year on a piece of land to build a chapel; lease dated 10/03/1875.
The early members would have been adults and even if baptised at Salem, would have been previously baptised in their local church.
Emyr