What better way to tell a lie than with a grin? Masks are so effective in Dunbar's poem because they appear to "grin." Sure it may look nice, but it's nothing more than a lie.
This debt we pay to human guile; (3)
Trillion dollar deficits are nothin' compared to the debt we all pay to "human guile." And how do we know the speaker is talking to all of us? Because the word "human" opens up interpretation in a much more universal way.
Nay, let them only see us, while We wear the mask. (8-9)
The world just doesn't get it because of the mask that "grins and lies." On the other hand, the world also chooses to remain ignorant, which we get from the speaker's inclusion of "let them."
We smile, but, O great Christ, our cries To thee from tortured souls arise.(10-11)
Since no one in the living world understands what's really behind that mask, the speaker needs to call in for some spiritual support. And since those souls are "tortured," we're hoping for speed dial.