Momentum is the most "interactive" of the Crusader's powers and is the one that usually gets most of the attention, but I consider it to be mostly incidental. It's nice and all, but it can be fiddly and isn't always useful.
The real action is with the extra altar and the condition immunity. The extra altar might seem unremarkable at first, but it is actually super strong. The deal with it is that it has a very high skill cap. God juggling is one of the most powerful strategies, but is also one of the most difficult to master. The odds of a strong conversion chain existing on the map are high, especially if you stack the Extra Altar prep on top of the Crusader's native power. Some simple two god chains to look out for are
JJ -> TT
GG -> TT
JJ -> Dracul
GG -> Dracul (strong early game into late game conversion with all four)
Binlor -> Earthmother (perversely satisfying when it works)
Binlor -> GG (explore most of the map under Binlor at 1st level and then convert into GG with tons of piety banked and all of your levels still available to be earned)
Mystera -> GG (similar deal, probably not quite as strong)
Taurog -> Dracul (easy piety with Taurog, strong end game with Dracul)
Taurog + Pactmaker (Affectionately know as "Packrog," take either Warrior or Body Pact and go nuts).
Earthmother +burn salve (use EM as a "free" desecration to supercharge any of the other combos)
This is not an exhaustive list, but hopefully gives you some ideas.
The condition immunity is also strong, but you can't be passive about it. Mentally subtract "half a level" from any monster that has poison, mana burn or curse, and recognize that you can challenge such monsters from a lower level than you usually can. Curse immunity also lets you make specialized resistance builds that can make even the Monk feel jealous.