Stafford said of his poetry that he must allow himself to be “willingly fallible” to deserve a place in the realm where “miracles happen" ... [the] quality of trust in the imagination ... [to] offer many surprises and provide a vivid picture of one man’s quest to learn “how to live.”(McGill's Survey of American Literature, Revised Edition on eNotes)
The answer to the meaning of the church imagery is tied to the first line of the second stanza: "Any wound is real." The first stanza sets forth imagery of fights and violence when "life ... slashes your face as a friend." The second stanza calls up secret wounds that are equally real. The expectation of meaning in this imagery associated with a woman in church is that of peace, worship, love, joyfulness.
Yet, when the light from the church window reveals a "lesson," this is a lesson in the hardships of life and of the endurance through hardships. This is the opposite of cliche church imagery of peace and tranquility; it is life's imagery of endurance through hardships. This has the meaning of a sorrow that goes so deep and cuts so deep that a "choir can't reach it when they sing."
This imagery calls up the almost mystical quality and ability music has to resonate with emotions and with the soul. Yet the meaning of the "lesson" on the woman's face is too deep for even music to resonate with. The third stanza deals a shock and a blow. The children's faces, in which most people see hope for the future and for love and kindness, reveal the "promise" of their own "scars" as seen in the light of the church window. The meaning here is that upon the perfect bloom of youth will fall the deepening scars of wounds and sorrows that "a choir can't reach."
Rows of children lift their faces of promise.
places where the scars will be.