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In Genesis 17 where we read the command to Abraham that every male child eight days old is to be circumcised (including the children of servants and slaves - even if not Abraham's descendants (verse 12), there is this unnerving penalty presecribed in verse 14:

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But an uncircumcised male ... shall be cut off from his people; he has broken My covenant.
An eight-day-old covenant breaker? It's not as though an eight-day-old could circumcise himself if his parents failed to do so - yet God's word is explicit.
Just to clarify (and I expect you agree), verse 14 says the child shall be cut off from his people, but not necessarily eternally from God himself (though I agree with you that in several cases many were probably cut off from God, but not “all”). Though a serious offense (Exodus 4:24-26), we cannot say that “all” children that were uncircumcised under the Old Covenant were unsaved: (1) your own example of John the Baptist was saved prior to circumcision, and (2) Israel had abandoned circumcision during Moses' leadership (Joshua 5:4-7), however Joshua did not just lead some elect individuals that were 7 days old and under.