The question is phrased to suggest that there is one unique way to summarize a poem (i. e. "THE summary"), but that is not actually the case. There are many different ways in which one can paraphrase or summarize a literary work, depending on the particular elements of the work one considers most important; one can create "a summary" but not "the summary."
The poem consists of four quatrains written in common measure, a meter often used in both ballads and hymns. The first and final stanza describe a certain type of winter afternoon light. The term "slant" suggests that the sun is low on the horizon, as would be true on a winter afternoon in New England.
The narrator compares this type of light to "Cathedral Tunes," suggesting solemnity and majesty, which the narrator also seems to find oppressive. The light is described as in some way causing pain, and being associated with despair and death. This association of deep winter with some sort of death or despair is fairly common in poetry, from Richard the Third's famous soliloquy concerning "the winter of our discontent" and John Donne's "A Nocturnal upon St. Lucy's Day" to Thomas Hardy's "The Darkling Thrush."
Rather than religion giving meaning to suffering, both religion and winter seem to symbolize some sort of inexplicable suffering to the narrator that descends out of the air. The conclusion of the poem could be seen to associate this winter light passing over the landscape and revealing death with that aspect of most religions that focuses on human mortality.
The poem has no distinct narrative shape, but as is the case with much of Dickinson's work, is suggestive, fragmented, and impressionistic, leaving room for multiple layers of interpretation.