Yeah, in the past, it was the Law of Moses, particularly circumcision, not water baptism, that separated Jews from Gentiles. Its quite obvious that water baptism would be no problems for most of the Gentiles who believe then to undergo, but circumcision was a different ball game altogether.
The issue of salvation in the OT was vague compared to under Paul's gospel.
But one aspect is quite clear: the Jews in the OT
were not saved individually (unlike us now) , but were saved by being part of the collective Israel.
Gentiles could be saved in the OT, but only thru the Jews (
Genesis 12:3) This means you have to be part of Israel, in order to be saved. But if you are cut off from the collective, there goes your salvation.
And under the covenant given to Abraham for all Jews,
circumcision was necessary for anyone, those who refused to be circumcised were immediately cut off. The passage is found in
Genesis 17
9 And God said unto Abraham,
Thou shalt keep my covenant therefore, thou, and thy seed after thee in their generations.
10
This is my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised.
11 And ye shall circumcise the flesh of your foreskin; and it shall be a token of the covenant betwixt me and you.
12 And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed.
13 He that is born in thy house, and he that is bought with thy money, must needs be circumcised: and my covenant shall be in your flesh for an everlasting covenant.
14
And the uncircumcised man child whose flesh of his foreskin is not circumcised, that soul shall be cut off from his people; he hath broken my covenant.
But thanks to the new gospel of grace given to Paul, which did away with circumcision and water baptism, the dividing wall between Jews and Gentiles was finally broken.