Here we want to focus in on Gentiles in the land, under the Law, and how it apples to this age (if at all).
We start with a little background.
The covenant of the Law is introduced 2400 years from Adam and 400 years from Abraham. It has to do with the land, the kingdom, and the priesthood. In this age, we are not "a kingdom of priests" and neither was Israel under the Old Covenant since they failed. This is part of the yet future New Covenant. It is all of the earth. Do you believe you are a priest? The you must answer the question: a priest for whom?
And Moses went up to God, and the Lord called to him from the mountain, saying, “Thus you shall say to the house of Jacob, and tell the children of Israel: ‘You have seen what I did to the Egyptians, and how I bore you on eagles’ wings and brought you to Myself. Now therefore, if you will indeed obey My voice and keep My covenant, then you shall be a special treasure to Me above all people; for all the earth is Mine. And you shall be to Me a kingdom of priests and a holy nation.’ These are the words which you shall speak to the children of Israel.”
This covenant is not given to "strangers." In the Law, unless one got circumcised, he remained a "stranger" [Gentile]. If he was in the land, he was not to pollute it, but that only applied to Gentiles in the land.
But, there are more than just Israelites in view in this passage. In THEIR [Israel's] kingdom, THEY will be PRIESTS. Priests for whom? Themselves? No, priests for the nations in the coming kingdom age. This promise has nothing to do with the period from Adam-Abram (the land) nor the period from Adam-Moses (the kingdom) nor with the Post Acts Calling in the heavenly places.
The Lord Jesus Christ forbade "the gospel of the Kingdom" to be preached to Gentiles or outside of Israel (Matthew 10:5-7). This is after he commends the centurion's faith and juxtaposes it with the "sons of the Kingdom" (Jews) in Chapter 8. But the kingdom was only to be preached to "the lost sheep of the house of Israel."
To whom does the Lord say he was sent? *“I was not sent except to the lost sheep of the house of Israel.” Is he a liar? No. Paul tells us in Romans 15:8, the Lord Jesus was sent "to confirm the promises made to the fathers." On Pentecost, Peter preached to Jews only. He made an offer to Jews only. The 12 (save once) preached to Jews only.
In Acts 21, we see Jewish believers keeping the Law (including circumcision) and the Gentile believers limited to the "necessary" things (cp. Acts 15:28) which are laid out in no uncertain terms.
Take them and be purified with them, and pay their expenses so that they may shave their heads, and that all may know that those things of which they were informed concerning you are nothing, but that you yourself also walk orderly and keep the law. But concerning the Gentiles who believe, we have written and decided that they should observe no such thing, except that they should keep themselves from things offered to idols, from blood, from things strangled, and from sexual immorality.”
In Romans 9, Paul speaks of the "grafting in" of Gentiles into the root of Israel.
There have always been believers not part of Israel from Adam on. They are separate. They do not come under Israel or the Law by faith (and neither do we). Solomos speaks of a believer who comes from outside Israel. Such a person is not subject the rules of the land. Gentiles observing with Israel are only those in the land, for the sake of the land. They have no promise of a priesthood, etc., but they cannot pollute the land.
Moreover, concerning a foreigner, who is not of Your people Israel, but has come from a far country for the sake of Your great name and Your mighty hand and Your outstretched arm, when they come and pray in this temple; 3then hear from heaven Your dwelling place, and do according to all for which the foreigner calls to You, that all peoples of the earth may know Your name and fear You, as do Your people Israel, and that they may know that this temple which I have built is called by Your name.
-2 Chron 6:32-33
I can't do an entire survey of the OT, but there are promises and warnings specific only to Israel and only in the land. Gentiles to whom Jonah preached in Nineveh, for example, since they did not live in the land, did not have to keep the Law. Daniel, outside the land, never told a Gentile to keep the law. It is land, kingdom, priesthood specific. And even then, a Gentile could not participate in the Passover unless he was circumcised (Ex 12:48; Num 9:14).
When the heavens are shut up and there is no rain because they have sinned against You, when they pray toward this place and confess Your name, and turn from their sin because You afflict them, then hear in heaven, and forgive the sin of Your servants, Your people Israel, that You may teach them the good way in which they should walk; and send rain on Your land which You have given to Your people as an inheritance.
There were believers under the Covenant in Israel and "strangers to the covenants and promises" yet believers. The Covenants (both Old and New) have always dealt with the land; always with the priesthood of Israel. And the New Covenant is yet future. It is not here. There is no priesthood. And if there were, a priesthood for whom?
The Case of Cornelius
Cornelius was not the first Gentile to have faith unto Life. We saw the Centurion in Matthew 8 and the Canaanite woman in Matthew 15 have faith. But the disciples were forbidden to preach the "gospel of the kingdom" or that the kingdom was "at hand" to Gentiles. That did not mean Gentiles could not have Life. And did the centurion or the woman (called a "little dog" separate from "the children") obey the Law? No.
What astonished the 12 about Cornelius was not that he believed, but rather that he received the same gifts of the Spirit as they (Jews). This was start of the "grafting in." They knew Gentiles could believe, they'd seen it in the earthly ministry of Iēsoûs Christos [Jesus Christ]. Cornelius was a believer when Peter arrived. He then had the Spirit fall and he displayed gifts (before he was baptized, btw).
Then Peter said to them, “You know how unlawful it is [present tense] for a Jewish man to keep company with or go to one of another nation [gentiles]. But God has shown me that I should [no longer] call any man common or unclean.
Then Peter opened his mouth and said: “In truth I perceive that God shows no partiality. But in every nation whoever fears Him and works righteousness is accepted by Him. The word which God sent to the children of Israel, preaching peace through Jesus Christ—He is Lord of all— that word you know...
The Canaanite woman in Matthew 15 (during the Lord's earthly ministry) addressed him as "Son of David." Gentiles do not have the right to address him that way. when a Jew addressed him that way, he listened. When the gentile woman used that title, the Lord ignores her and compares her to a little dog.
We saw that Gentiles living outside the land were never preached the Law nor the Gospel of the Kingdom. We that the Lord forbade his disciples from doing so. These are not what we would call "salvation" truths. They are obedience and maturity and reward truths. They are connected to the promises of the land and the Kingdom in Israel as we have seen.
So, while in the land, a stranger could live among Israel, but he was required to observe the things pertaining to the earthly. These are the "earthly ordinances" of the hope of the Kingdom.
We must distinguish between these two enemies of the Christian: things that are "worldly" (which appeal to the lower nature of the flesh) and thins that are "earthly" (which appeal to the religious nature and pride of the believer). Both can rob you of your reward. We must walk according to the calling to which we have been called (Eph 4:1).
Keep the Law
Why does the Lord tell the young ruler to keep the law to enter into life, he is speaking of the Kingdom on earth, not resurrection life in the far above the heavens. Matthew 19
Now behold, one came and said to Him, “Good[Teacher, what good thing shall I do that I may have eternal life?”
So He said to him, “Why do you call Me good? No one is good but One, that is, God. But if you want to enter into life, keep the commandments.”
He said to Him, “Which ones?”
Jesus said, “‘You shall not murder,’ ‘You shall not commit adultery,’ ‘You shall not steal,’ ‘You shall not bear false witness,’ ‘Honor your father and your mother,’ and, ‘You shall love your neighbor as yourself.’ ”
The young man said to Him, “All these things I have kept from my youth. What do I still lack?”
Jesus said to him, “If you want to be perfect [mature/qualified], go, sell what you have and give to the poor, and you will have treasure in heaven; and come, follow Me.”
But when the young man heard that saying, he went away sorrowful, for he had great possessions.
The Lord does not contradict that the man kept the commandments listed and adds a layer of requirement to test his heart. Is the Lord teaching "Justification by Law Plus Philanthropy" (we note "justification" as well as "salvation" are context dependent)? No. The Lord is still preaching the coming Kingdom. This is especially true in all of Matthew.
We see the Lord reference the "gospel of the Kingdom" (which he preached in Matthew 4 and 9) in regard to Israel and the Tribulation in Matthew 24.
But he who endures to the end shall be saved. And this gospel of the kingdom will be preached in all the world as a witness to all the nations, and then the end will come.
We do not believe this is "justification by endurance." This is physical salvation during the Tribulation. They must endure both in body and spirit. This does not mean martyrs miss out on anything. Quite the contrary. But he is about to tell "those who are in Judea flee to the mountains." The context is heeding his warning with promise of deliverance if they endure.
Remember, the gospel of the kingdom was to be preached to JEWS ONLY. When the Lord returns to rescue faithful Israel and take David's throne in the land THEM the gospel of the Kingdom will be preached by Israel to the nations [gentiles].
All of this is predicated on understanding God's different plans for different companies of believers and understanding the different between God's plans for earth, the New Jerusalem, the new earth, and the far above the heavens.
Adam looked for Paradise restored. Abraham looked for the New Jerusalem. David looked for his seed to sit on an age-enduring throne. Israel looks for its promises in the land and kingdom and as a Kingdom of priests. We look for a calling with is not earthly, but rather it is in the "far above the heavens where Christ dwells."
If we muddle up those different callings, we'll miss our calling and our reward.