Those people didn't/don't exist when Israel was an earthly kingdom. When Israel did exist, before the new covenant, being considered one of God's people was being joined to them through the covenant of circumcision. Apart from women who could not be circumcised, I don't think you can show me a scriptural exception. Quite the contrary. Paul says that the gentiles were lost and without hope.
Being one of God's people has always been through faith. Circumcision was but one part of the obedience that comes through faith - the first step (for a male) in obeying the whole of the law, which was required for natural Israel. Other simple examples (for all, not just males) were not committing adultery, and not murdering. The easiest demonstration that circumcision was not the be-all-and-end-all when it came to natural Israel is righteous women, who obviously couldn't be circumcised. I've given examples of old Testament saints who were around before the circumcision covenant, and Abraham, who was explicitly made righteous for believing God while yet uncircumcised.
The closest example of a non-Israelite believer I can think of was Naaman. Note that Elisha didn't say to Naaman - "Oh, before you go, you better quickly get circumcised too". He simply said "Go in peace". God doesn't save us because of our works. It's possible Naaman started to obey more of God's law later on (sanctification), but I find it hard to believe a man of God would say to an unbeliever who had just professed a wish to become a worshipper of the true God to "go in peace", if the recent unbeliever was still damned until he was circumcised.
2 Kings 5:11 - 19
But Naaman was wroth, and went away, and said, Behold, I thought, He will surely come out to me, and stand, and call on the name of the Lord his God, and strike his hand over the place, and recover the leper.
Are not Abana and Pharpar, rivers of Damascus, better than all the waters of Israel? may I not wash in them, and be clean? So he turned and went away in a rage.
And his servants came near, and spake unto him, and said, My father, if the prophet had bid thee do some great thing, wouldest thou not have done it? how much rather then, when he saith to thee, Wash, and be clean?
Then went he down, and dipped himself seven times in Jordan, according to the saying of the man of God: and his flesh came again like unto the flesh of a little child, and he was clean.
And he returned to the man of God, he and all his company, and came, and stood before him: and he said, Behold, now I know that there is no God in all the earth, but in Israel: now therefore, I pray thee, take a blessing of thy servant.
But he said, As the Lord liveth, before whom I stand, I will receive none. And he urged him to take it; but he refused.
And Naaman said, Shall there not then, I pray thee, be given to thy servant two mules' burden of earth? for thy servant will henceforth offer neither burnt offering nor sacrifice unto other gods, but unto the Lord.
In this thing the Lord pardon thy servant, that when my master goeth into the house of Rimmon to worship there, and he leaneth on my hand, and I bow myself in the house of Rimmon: when I bow down myself in the house of Rimmon, the Lord pardon thy servant in this thing.
And he said unto him, Go in peace. So he departed from him a little way.