"it is become" v.s. "it becomes", which one is correct?
The verb "is", is stative. "Become" denotes a change. "Is become" is therefore an oxymoron.
However, one can say "has become" which is a past event that continues to the present.
The "to-be" verb is also used as an auxiliary with a participle as in "is becoming."
It is likely that "is become" is archaic but not grammatical today.