Yes, PIETISMS is a Scrabble word!

Letter values in Scrabble US: P 3, I 1, E 1, T 1, I 1, S 1, M 3, S 1.

Is it a valid word?

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

17th and 18th-century German movement in the Lutheran Church stressing personal piety and devotion
exaggerated or affected piety and religious zeal

Not playable in Scrabble US: RELIGIONISM, RELIGIOUSISM.