NOTE: The character talking was a disciple of the daikaioushin.
What is the だって doing in this sentence and how should I be translating or understanding it?
だって = でも , "even"
This sentence would appear to mean literally "So long as I continued to be in the condition of a disciple of Daikai-oshinsama . . . even I was able to use the power of revival/resurrection". Less literally "Even I, so long as I remained a disciple of Daikai-oshinsama, retained the power of resurrection".
Note that while である= "is", でいる= "continues to be", "remains".