Yes, APOSTLE is a Scrabble word!

Letter values in Scrabble US: A 1, P 3, O 1, S 1, T 1, L 1, E 1.

Is it a valid word?

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

an ardent early supporter of a cause or reform
any important early teacher of Christianity or a Christian missionary to a people
(New Testament) one of the original 12 disciples chosen by Christ to preach his gospel

Also: apostelic father.