"That
at that time ye were without Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise." - Ephesians 2
Paul was saying that there was a time in the past (before Christ) when an uncircumcised person was not an inheritor of the promise, i.e., aliens (non-members) of Israel but, in Christ, that is no longer the case, because in Christ circumcision of the flesh becomes meaningless.
With Christ, what matters is circumcision of the heart, which is attained through faith. In fact, the original value of circumcision of the flesh wasn't in the cutting of the flesh anyway. It's value was as a symbol of faith. Circumcision without faith was worthless, but faith without circumcision of the flesh is 100% sufficient for membership in the commonwealth of Israel.
Furthermore, the Ephesians quote above makes clear that
only those who believe in Christ are in any real sense part of Israel, i.e., being without Christ makes one a non-member of Israel.
"The lost sheep of the house of Israel" in Matthew 15:24, therefore, refers to
all who were outside of Israel (i,e., all who rejected Christ), whether descended from Abraham or not. That's what made them lost. He sought to bring them in, which could only be accomplished by faith in him.
