The "Awful Sin" in McKay's "The Lynching"

After reading and reflecting on Claude McKay's poetry, I found that I was most impressed, but also perplexed by McKay's poem "The Lynching." Most particularly, I am confused by the meaning behind line 4 in which McKay states in context to an African American man being lynched, "The awful sin remained still unforgiven" (707). What is this sin in which McKay is referring to? McKay doesn't allude to the man having committed any sort of crime or wrong and doesn't include any kind of description of the man in the poem. Adding to my confusion is the footnote below the poem that states that the first four lines of the poem allude to the crucifixion of Christ. What does this context mean in reference to the lynchings that occurred in the South? After reading the poem again, I noticed another curious aspect of the poem in the first two lines; the word spirit is capitalized and the word father, referring to God, is not: "His Spirit in smoke ascended to high heaven./ His father, by the cruelest way of pain,/ Had bidden him to his bosom once again" (707). What is the meaning behind this use of capitalization and how does it explain line 4 of the poem? After some thought and taking all of these aspects of the poem into account, I think perhaps that the "awful sin" mentioned in line 4 is referring to the Biblical sin of mankind or is actually not referring to a particular sin at all and is merely using this phrase as a way to express cynicism and the irony behind lynching people for the color of their skin alone. I think that the "awful sin" could be the Biblical sin of mankind due to the fact that the first four lines are referring to the crucifixion of Christ and that McKay seems to be drawing parallels between Jesus and the lynched man of the poem. Although this could be McKay's intention, I tend to think that the "awful sin" is alluding to McKay's cynicism and the irony of skin color being a "sin." This interpretation makes sense in context to the rest of the poem as McKay's choice not to capitalize father could be seen as disrespectful and shows a bitterness towards God. His bitterness is also reflected in lines 6 and 7, as the "star" (God) that had been guiding the lynched man "gave him up at last to Fate's wild whim" (708). Whether the "awful sin" is interpreted as the sin of mankind or as an expression of irony, McKay's poem powerfully reflects the horrors of lynching and expresses the author's questioning faith in the face of great injustice and tragedy.