38 And Mary said, Behold the handmaid of the Lord; be it unto me according to thy word. And the angel departed from her.
According to Merriam and Webster;
be
verb \ˈbē\
—used to indicate the identity of a person or thing
—used to describe the qualities of a person or thing
—used to indicate the condition of a person or thing
It is a (pre-existing) state of being and not a "let it be" future tense
Stealing that for future use