Like all tests in Burning Wheel, Task and Intent are the right place to start.
Firstly, does this Intent make sense for your game? Is the character going through the motions to inspire confidence or are they actually invoking the Divine? Are the Faithful common or rare, and does the Divine often intercede for this sort of prayer?
Secondly, assuming the Intent makes sense, it's not going to be mechanically making the same roll 20 times.
Tests must be distilled down to as few rolls as possible. (Let It Ride, page 32.)
Finally, Faith is more open-ended and powerful than most of the other skills and abilities in the game. The list of example prayers provides some guidance, but the Ob 3 Blessing example affects only one person or thing, so that same Ob probably shouldn't affect the entire squad. If this Intent makes sense in your game, then the Task should be a Faith test with a higher Obstacle.
Now we're down to nailing down the Obstacle and also the exact Intent. What benefit are they asking for and who is it affecting? One of the options from Blessing (page 524) but extended to the squad for the duration of the intent? As the GM, is this crossing going to be a whole session, a series of linked test, or one big test? Which roll or rolls will this blessing affect? How will the blessing fictionally manifest? Is this about literal climbing or a fight during the crossing?
For example, if it's mostly about the mechanical climb and that's going to be a linked test, I'd increase the Ob to 5 and the effect is +2D to be used or portioned across one or two tests in the linked test.
If this is going to be scripted combat, sounds like Range & Cover? I'd again go with Blessing—+1D to one skill or stat for the conflict—at Ob 5 to cover everyone in the squad. For Bloody Versus, figure out if it's Attack, Defend, or both that are affected—the nature of the Divine and the Faithful's idiom should be reflected here—but Ob 4 or 5 for +1D or +2D seems reasonable.
