And if one of the spouses is wanting to become closer to Christ while the other isn't, when would divorce be permissible?
Someone has already posted St Paul's words, which are directly applicable to this situation.
To add something from the canonical tradition: It is possible to divorce one's spouse (and remain in good standing in the Church) if that spouse was a believer but later becomes a confirmed "apostate." Please note: Possible
, not required or even advised.
In previous centuries, "apostasy" meant leaving the Orthodox Church. In the situation presented by the OP, most spiritual courts would probably consider it "apostasy" if the formerly Christian spouse became a non-Christian (or even a member of a non-Trinitarian sect, e.g. Jehovah's Witness, Mormon, Christian Scientist -- none of which can be married to an Orthodox Christian in the first place).
Make sense? In other words, it doesn't matter how "spiritual" or "observant" or supportive of your spiritual journey one's spouse is -- these are all qualitative, private issues between spouses -- but rather what one's public confession of faith is, and how the spouse backs that up liturgically. If they start confessing and worshiping another god, gods, or set of gurus, then there are grounds for divorce. If they just aren't particularly zealous or interested in following Jesus, that's a matter for prayer and patience.