James blunt your are beautiful裡面的歌詞，l saw an angel of that I m sure.請問這裡的of that 的詞性跟用法是怎樣？感謝了！?

I saw an angel.--On rearranging "of that I'm sure"="I'm sure of that" ;
I am sure of that--(that) is relative pronoun used in defining the clause--"saw an angel;" often preferred to "that" for person=an angel ;and often used in place of "who" as "who was an angel."