Yes, FAITHFULS is a Scrabble word!

Letter values in Scrabble US: F 4, A 1, I 1, T 1, H 4, F 4, U 1, L 1, S 1.

Is it a valid word?

FAITHFULS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

any loyal and steadfast following
a group of people who adhere to a common faith and habitually attend a given church