From the
Pasuk , כי אל אשר תלכי אלך we see that Rus was
Megayer after the death of her first husband.
If that is
true then we need to understand why was it necessary
to be Meyavem Rus? The first marriage was invalid.(she wasn't Jewish)
Answer I
heard from Mori V’rabi Hagaon R. Shlomo Miller Shlita:
Rus was
Megayer before her first marriage. This Giyur was assumed to be invalid
because she accepted only 612 Mitzvohs.
She had not
accepted The Mitzvah of not marrying a Moavi.
It
was not yet known that a female Moavi may marry a Yisroel.
After the
death of her husband she was Megayer a proper Giyur and
accepted all 613 Mitzvohs. (כי אל
אשר תלכי אלך)
Once Chazal were Mechadesh the Halacha מואבי
ולא מואבית
this validated her first Giyur retroactively . She hasn’t
rejected
a single Mitzvah and therefore her marriage was valid and required
Yivum.
