This looks like an extract from poetry. Here, the last two lines are meant to be read together. They mean, "I am sure that I saw an angel."
So, nothing is omitted.
A change in punctuation, putting a comma after the word "angel", would have made it easier to understand. However, it seems that the writer/poet wants to increase emphasis by putting in the full-stop.
Nothing is omitted in "Of that I'm sure." This is a case of inverting elements of a sentence in poetry. The uninverted form is "I'm sure of that." But to place the word "sure" at the end, where it will create a rhyme with "pure", the author has changed the usual word order.