Hello I searched for David in the 1881 census there is one match for his name (too old to be yours):
Dwelling: Lower Village
Census Place: Whitchurch, Glamorgan, Wales
Source: FHL Film 1342268 PRO Ref RG11 Piece 5275 Folio 126 Page 2
Marr Age Sex Birthplace
David SALATHIEL M 29 M Whitchurch, Glamorgan, Wales
Rel: Head
Occ: Smith Striker
Selina SALATHIEL M 34 F Bridgend, Glamorgan, Wales
Rel: Wife
Daniel SALATHIEL 7 M Whitchurch, Glamorgan, Wales
Rel: Son
Occ: Scholar
George SALATHIEL 3 M Bridgend, Glamorgan, Wales
Rel: Son
Occ: Scholar
Maria SALATHIEL 7 m F Whitchurch, Glamorgan, Wales
Rel: Daur
Henry EVANS U 60 M Whitchurch, Glamorgan, Wales
Rel: Boarder
Occ: General Labourer
The only Salathiel with links to Caerphilly is this one:
Dwelling: Lower Village
Census Place: Whitchurch, Glamorgan, Wales
Source: FHL Film 1342268 PRO Ref RG11 Piece 5275 Folio 127 Page 3
Marr Age Sex Birthplace
Daniel SALATHIEL M 63 M Caerphilly, Glamorgan, Wales
Rel: Head
Occ: Boot & Shoemaker
There are 72 matches to the surname in the 1881 census.
Going onto Mary Snell, there are 9 matches to the name in the 1881 census:
SNELL, Mary Wife <1837> Birth: Devo
Census: Monm Bedwellty
SNELL, Mary Wife <1842> Birth: Monm Ponthir Llangattock
Census: Monm Llangattock
SNELL, Mary Jane Wife <1844> Birth: Lond London
Census: Monm Aberystruth
SNELL, Mary M. Wife <1849> Birth: Berk Kinterbury
Census: Glam Swansea Town
SNELL, Mary Wife <1855> Birth: Pemb Fishguard
Census: Glam Swansea Town
SNELL, Mary Jane Rel <1863> Birth: Monm Abercarne
Census: Monm Mynyddyslwyn
SNELL, Mary Dau <1868> Birth: Monm Christchurch
Census: Monm Christchurch
SNELL, Mary Dau <1869> Birth: Monm Llanvrechva Lower
Census: Monm Lower Llanvrechva
SNELL, Mary Dau <1870> Birth: Glam Gelygaer
Census: Glam Eglwysilan
As for Twyn C.M Chapel. This may help you:
QUOTE: "Tuesday, December 6.
Notwithstanding the rain and the knowledge that the chapels were far too small to accommodate any great numbers, the crowds who came to
Caerphilly to bear and see, or try to hear and see, Mr. Evan Roberts to-day were just as large as they were on the previous day. The sights in the open-air were not so manifest to the casual observer, perhaps, because, instead of being brought together into the square near the
Twyn Chapel. the people were scattered about, the meetings being held in various chapels at the other end of the town, and there was, fortunately, some uncertainty as to where the missioner would speak, or whether he would deliver addresses at more than one place of worship.
http://www.sendrevival.com/history/welsh_revival/religous_revival/chapter_22.htmHope some of this helps you in your search