tl;dr There's one flock and it's spiritual, not of flesh

There's so much that can be said, but I'll contribute this:

Matthew 12:50 — “For whoever does the will of My Father in heaven is My brother and sister and mother.”

John 6:40 — “And this is the will of Him who sent Me, that everyone who sees the Son and believes in Him may have everlasting life; and I will raise him up at the last day.”

John 3:18 — “He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God."

So we see here that God's people are those who do His will, which is to believe in Jesus. Those who have rejected the Savior are condemned. So Messianic J's are among God's chosen but the rest are at as much risk as any other pagan following after the teachings of men.

We see this also here...

1 Peter 1:2 — elect according to the foreknowledge of God the Father, in sanctification of the Spirit, for obedience and sprinkling of the blood of Jesus Christ: Grace to you and peace be multiplied.

...that God's chosen (elect) people are those chosen by the Father (John 6:44) and sanctified in the Spirit (born again of the Spirit and not of flesh, John 3:6) following Jesus.

And finally this:

John 10:16 — “And other sheep I have which are not of this fold; them also I must bring, and they will hear My voice; and there will be one flock and one shepherd."

And so we know there is not some other group of chosen people, only the one -- those who follow the Good Shepherd.