Yes, APOSTASY is a Scrabble word!

Letter values in Scrabble US: A 1, P 3, O 1, S 1, T 1, A 1, S 1, Y 4.

Is it a valid word?

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

the state of having rejected your religious beliefs or your political party or a cause (often in favor of opposing beliefs or causes)
the act of abandoning a party for cause