Lyric discussion by BSCHAAF 

Cover art for A Sun Came lyrics by Sufjan Stevens

Not that it's incorrect to find literal meaning in the line "a sun came" but it just occured to me that he could be meaning a son came. The rest of the song points to Jesus Christ, the son of God, coming.

you could even add an extra word in the last three lines:

There is still a Lord but I never felt so lonely UNTIL a sun (son) came.