Four short lines, and the whole poem rests on one comparison: winter is a church. It isn't a crowded church, though. It's "alone" and "undressed," stripped of leaves, color and people. The image suggests bare trees and an empty landscape, a season with nothing left to hide behind.
What makes the picture warm rather than bleak is the last line. This empty church is "making a life of music." The word "ecclesiastical" in the title points toward worship, and the poem seems to hear winter's sounds as a kind of hymn: wind, creaking branches, perhaps birdsong or the hush itself. Judging by the imagery, the speaker isn't mourning the cold season but listening closely to it.
The mood is quiet, reverent and a little solitary. There's loneliness in "alone," but also dignity, even contentment. The speaker seems to find that emptiness leaves room for something else to be heard, and that a stripped-down season can still be full of life.