what happens if opponent plays rebuke with a reflection token Altar to Unity

Yes — if the Rebuke is legal, the Reflection token is simply gone: it moves to its owner's hand and then ceases to exist immediately, so it never comes back (FAQ #122). Copying a unit with Deceiver doesn't change that it's a token, not a card, and tokens can't exist in any non-board zone.

Two things to check before this even happens, though. First, Rebuke reads "Return a unit at a battlefield to its owner's hand" — the 1 Might Recruit that Altar to Unity makes is played to your Base, so it isn't a legal Rebuke target while it sits there; the Reflection token made by Deceiver at a battlefield is. Second, Rebuke is an [Action], so your opponent can only play it on their own turn or during a showdown — they can't use it to respond to Altar to Unity's hold trigger, since a triggered ability on the chain closes the state and only [Reaction] cards may be played.