If there is no exiled card as Bishop of Binding’s last ability resolves, most likely because Bishop of Binding left the battlefield while that ability was on the stack, X is 0. The same is true if the exiled card doesn’t have a power, most likely because it’s a noncreature card that had become a creature.

Auras attached to the exiled creature will be put into their owners’ graveyards. Any Equipment will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist.

If a Vehicle is exiled with Bishop of Binding, use its printed power to determine the value of X.

If a token is exiled this way, it will cease to exist and won’t return to the battlefield.

If Bishop of Binding leaves the battlefield before its first triggered ability tries to resolve, the target creature won’t be exiled.

The value of X is determined only as Bishop of Binding’s second ability resolves. It won’t change if the card leaves exile later in the turn.