If you're wondering, "what ruined church?" we recommend a little rewind and a closer look.
The ruined church shows up a few times in the Platoon, most crucially as part of the battle in which Elias dies. Its significance? Not so difficult to divine. (Pun intended).
We first see it when the platoon is on patrol in the rain, about to reenter the bunker complex they discovered. Taylor is describing the civil war raging in the platoon, the divide between Barnes's and Elias's men. Right at the moment, a ruined church appears, as if to offer itself as the perfect for the platoon's own destroyed faith in itself, in any idea of harmony.
We see the church again when Elias is actually getting shot by NVA troops. In fact, he's not that far from it. It's almost like he's trying to get to it, to find some hope in God before he finally dies. The reappearance of the church also drives home the message: the platoon, and the ethos that formerly held it together—it's "religion"—is gone, given a final send-off in Barnes' murder of Elias.
We see the church one final time, when Taylor and the platoon are sent back out (it's not clear where). The destroyed church lends an ominous significance to the film's final movement, the suggests that the platoon may finally meet its very literal end (many of the guys end up dying).
While the church has a particular resonance in each of its three appearances, it also symbolizes or suggests something more general about the war. If a church normally symbolizes religion, faith, and god, a destroyed church suggests just the opposite: a godless wasteland, a place where all hope and faith has fled. That's not being too extreme, for that is exactly how Vietnam, and the war, appear to the men of the platoon, a losing effort where even the best of guys (Chris Taylor) find it hard to do the right thing.