G-d ordained the practice when he changed Avram to Avraham.
And again with the souls Avraham 'made' in Charan.
And again at Mount Sinai with the mixed multitude
And again with Jethro in Bamidbar.
And again 30+ more times in Moshe's teaching in the desert reiterating that the ger is burdened with the law.
And again in Ruth when Boaz performed the right of levirate marriage for Ruth. We can safely assume her children were also considered Jewish as her great grandson David became King.
And again in Isaiah.
Surely a righteous gentile knowledgeable of their bible would be aware of these things; it is not what could be considered a cryptic concept....
Sometimes I wonder if gentile bibles were actually ever supposed to be read or are they just for decorating hotel rooms......
Some people would say you can only be Jewish if you are born a Jew, others would say that converts are indeed Jewish. It has to do with the Judaism being both a religion and a culture.
The law? G-d set that law when he called upon the pagan man Abram to follow G-d and only G-d and changed his name to Abraham. G-d set the law when the pagan Moabite woman Ruth was recognized as Jewish when she accompanied her mother-in-law to care for her, proclaiming that Naomi's people, family and G-d would be her G-d even though she wasn't obligated to care for Naomi. G-d set the law when He commanded that potential converts be welcomed and treasured.
Jews don't seek out converts but they are welcomed if they are willing to do the work and are truly called to take on the commandments (300+) and the irrational hatred.
Like NK there. Some are brave on the internet and cowardly in under their sheets because they know we fight back
Ruth, who has an entire Biblical book named after her, is originally a foreigner, a Moabite, and a convert, or a Jew-by-Choice, and from her is descended no less a person than King David. Think of that, according to Jewish tradition, King David, and all of his descendents, of whom tradition includes the yet-to-come Messiah, they all originally come from someone who converted to Judaism.
Today it mostly happens when a Jew marries a gentile. In the Bible, Ruth became an Israelite. So did Rahab, and probably Uriah. The prophet Jonah was sent specifically to preach to the non Jewish people of Nineve to convert and worship the true God.
In practice Judaism of today is not very mission orientated. It is as much an ethnic group and culture as an actual faith, though it differs a lot between individuals. Some people are even atheists or agnostics, yet identify themselves as Jews.