Whenever our Human Cleric friend or another of our creatures enter the battlefield under our control, we gain 1 life. We have no choice in the matter. It is not a may effect. Just gain the one life. There's nothing wrong with this ability. Of course, when you compare it to the famous "Soul Sisters" Soul Warden and Soul's Attendant (or even Auriok Champion if she's more your fancy), it's pretty underwhelming. The Soul Sisters give you one life whether it's your creature or an opponent's. And those gals cost only a single mana to cast. This dude costs 3 - 2 and a White.
However, for a cost of 2 mana and sacrificing another permanent of yours, you can transform him...
"...and I will prove my devotion." the Wayward Disciple says.
How exactly does the Wayward Disciple prove his devotion? By becoming a 2/4 clone of Zulaport Cutthroat. Seriously, he does exactly the same thing. Does this make him bad, having to essentially pay 3 more mana plus the sacrificing of one of your permanents to get what the Cutthroat can give you for just 1 and a Black? Many players seem to think so.
While he may not be in the realm of "Constructed playable" land, I do think that this flavorful dude can do some work in Drafts and sealed deck tournaments. You know, there are plenty of cards designed for just that environment. This is one of them. Also, Ayli, Eternal Pilgrim has become a fairly popular Commander who loves having more Clerics around. Being both White and Black, it's no problem for Pious Evangel and his flipside to be jammed in there. I mean, in Commander, Blood Artist is better. But maybe you don't want to pay $4 a copy for one (yeah, seriously $4). This is a cheap alternative, albeit one that makes you do a lot for it. And Blood Artist is actually better, since his ability activates when anyone dies.
Still, I love the flavor of Shadows Over Innistrad, and this guy is a win from a flavor perspective. But yeah, I'll stick with Zulaport Cutthroat, Blood Artist and the Soul Sisters in my Commander decks. Nice try my Evangel friend, but your devotion will not sway my faith.