Because the Holy Ghost over the bent World broods with warm breast and with ah! bright wings. (13-14)
The dove has long been a symbol of the Holy Ghost, as well as a symbol of peace. In a way this is a comforting idea. The poet says that the reason the sun still rises and sets is because we have not been abandoned. The "ah!" implies a deep satisfaction with this idea. Like the first line of the poem, the last line speaks directly about religion.