A.T. Robertson Commentary Luke 1:8

"Now it came to pass, while he executed the priest`s office before God in the order of his course," — Luke 1:8 (ASV)

While he executed the priest's office (εν τω ιερατευειν αυτον). A favourite idiom in Luke, εν with the articular infinitive and the accusative of general reference where the genitive absolute could have been used or a temporal conjunction and finite verb. It is proper Greek, but occurs often in the LXX, which Luke read, particularly in imitation of the Hebrew infinitive construct. The word ιερατευω does not appear in the ancient Greek, but in the LXX and this one example in Luke. It is on the Rosetta Stone and the early inscriptions so that the word was simply applied by the LXX translators from current usage.