What religion was Isaac?

Answer 1

What we now call Jewish. Isaac (Yitzhak) was a son of Abraham (Genesis ch.21); and God said that His covenant with Abraham would continue with Isaac (Genesis 17:18-19). Isaac had facial features that were identical to Abraham (Rashi commentary, Genesis 25:19).

God tested Abraham by asking him to offer Isaac as an offering (see Genesis ch.22); and this readiness for self-sacrifice bestowed a special holiness upon Isaac (Talmud, Yevamot 64).
Isaac married Rebecca (Genesis ch.24), his second cousin. Rebecca gave birth to two sons, Esau and Jacob (Genesis ch.25).

God spoke to Isaac, confirming His covenant with him (Genesis ch.26).
Despite harassment from the Philistines, Isaac enjoyed great success (ibid.). He reopened all the wells which his father had dug and the Philistines had stopped up (ibid.). The king of the Philistines, unable to ignore Isaac's phenomenal success, petitioned him to make a mutual treaty of non-aggression (ibid.).

In his advanced years, Isaac sought to bless his elder son Esau (Genesis ch.27). Rebecca took pains to thwart this and to ensure that Jacob would receive the blessing, since she had learned through prophecy (Genesis 25:23) that Jacob would be more favored by God. (See: Isaac's blessing)

Later it became apparent that Isaac admitted to Rebecca's having done the right thing, since he blessed Jacob again (Genesis 28:1-4) even after he found out what had happened (see Genesis 27:6-16).

Isaac sent Jacob to the ancestral family seat in Mesopotamia to choose a wife (Genesis ch.28). Though Jacob didn't return for over twenty years (Genesis 31:38), God granted Isaac extra years, so that he lived on for two decades after Jacob's return.
Isaac was buried next to his father Abraham, in Hebron (Genesis 49:31).