When We (first) sent to them two apostles, they rejected them.
This "when" is a conjunction.
from MacMillanWhen can be used in the following ways:
as a conjunction (connecting two clauses): When he saw me, he waved.
as a question adverb (introducing a direct or indirect question): When should we meet? ♦ Do you know when these houses were built?
as a relative adverb (referring back to a noun and introducing a relative clause): I remember the day when the war began. (introducing a clause that is the subject, object, or complement of another clause): My proudest moment was when I received the poetry prize.
Question adverbs or connective adverbs are canonically interrogative adverbs (when, where, why, how) used to introduce an embedded, or indirect, question, e.g. 'why' in
He asked why I had done it. -- why = adverb + linker
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