Actually, this probably doesn't belong at common.

I'd say definitely not. By default it's a Glorious Anthem/Gaea's Anthem/Crown of Convergence/etc for your turn only; those are virtually always rare (there was the odd Kabira Vindicator at uncommon). It also turns Brainstorm into Overrun, which is kinda awesome but also terrifying. Rare is the right place for this effect.

You know, this is a funny argument that comes up again and again. Crusade could easily be common. I think the reason why it was made a rare way back in Alpha is because it is heads and shoulders better than Holy Strength, not because it is confusing, or narrow. Unfortunately, to make a Crusade effect a common, we would have to fight past a large number of people who don't think it feels right.

If you make the bonus +0/+1, though, it suddenly becomes common again, (funny, that) so I'd suggest that. Also, when taking a step back, I have to admit that if one was to try to make Crusade common, they certainly wouldn't start with a card as potentially explosive as this one.

I've had this argument before, too. I really WANT to have crusade cards at common a) so they're easy to get hold of and b) so it's a good limited archetype. However, I was persuaded by the nay-sayers, that the problem with a crusade-like at common, is that it's likely to warp the limited game too much, making it all about "who can get the most of them". (Although, I mean, I'm not positive, it sounded convinicng but I'm open to being persuaded I was wrong.)

But yeah, this particular one is several notches more involved than that: it screams "splash me into a blue deck and see what you can get away with!". In other words, a build-around-me, which could be uncommon (?).

I don't know, but working from those premises, the qualities for a crusade-effect in common would be:

(i) sufficiently narrow that it's interesting if you can make a deck around it (in limited or constructed) (ii) but not so narrow it's completely useless without a dedicated deck (iii) not so high variance that it tends to win-or-lose games immediately

On the one hand, that suggests a much less sexy common-crusade. On the other hand, it suggests it may, in fact, be possible...?

Despite the fact that I made this at common, I really don't think it or other "Crusade" effects should be at common. I think it belongs at uncommon or even rare.