I disagree. We see at the fall of man that God promises a redeemer to Adam and Eve and their descendants, there was no Jew or Gentile then. I would agree that the Jews were made special partakers of promises unique to them, but I also believe that many (not all) of these promises extend to believers in general.
There were several things missing in the days of Adam that are present with us today. You were right to say there were no gentiles. That is certainly true but likewise there were no "nations." The nations were God's idea and he instituted this means of law and order in the middle of the second millennium and after the first global age.
Why did God then divide men into nations according to families and separate them with water when he divided the continents? It was to make sinful men responsible to authority. That is the first reason.
1Pe 2:13 ¶ Submit yourselves to every ordinance of man for the Lord’s sake: whether it be to the king, as supreme;
14 Or unto governors, as unto them that are sent by him for the punishment of evildoers, and for the praise of them that do well.
15 For so is the will of God, .......
The second reason is because separation and division among men ensures that they have the best chance of being saved. I did not say that, God inspired it, Paul gave voice to it, and Luke wrote it down. See here;
Acts 17:22 ¶ Then Paul stood in the midst of Mars’ hill, and said, Ye men of Athens, I perceive that in all things ye are too superstitious.
23 For as I passed by, and beheld your devotions, I found an altar with this inscription, TO THE UNKNOWN GOD. Whom therefore ye ignorantly worship, him declare I unto you.
24 God that made the world and all things therein, seeing that he is Lord of heaven and earth, dwelleth not in temples made with hands;
25 Neither is worshipped with men’s hands, as though he needed any thing, seeing he giveth to all life, and breath, and all things;
26
And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds (the boundaries) of their habitation;
Why?
27
That they should seek the Lord, if haply they might feel after him, and find him, though he be not far from every one of us:
28 For in him we live, and move, and have our being; as certain also of your own poets have said, For we are also his offspring.
29 Forasmuch then as we are the offspring of God, we ought not to think that the Godhead is like unto gold, or silver, or stone, graven by art and man’s device.
30 And the times of this ignorance God winked at; but now commandeth all men every where to repent:
31 Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead.
Go back now to the time before the flood between Adam after the fall and Noah and find a single law that God gave them except the consciousness of good and evil. The only one who had a law from God was Adam and he had only ONE law. Do not eat of the tree of the knowledge of good and evil. Even a murderer had no condemnation for his act of killing his brother because there was no law against it and the murderer was protected by God. You want to talk about grace.
This is particularly significant when you consider God later gave his law through Moses to Israel where a man was put to death for gathering sticks on the sabbath because God had given a law against it.
We have much commentary on this long after we have been given much more knowledge of God.
Ro 5:12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
This is physical death he is talking about in verse 12. The graveyards prove this is true.
But watch this and understand that most all people who reads it does not believe it. I doubt you will believe it.
13 (For until the law sin was in the world: but
sin is not imputed when there is no law.
14 Nevertheless death reigned
from Adam to Moses, even over them that had not sinned after the similitude of Adam’s transgression, who is the figure of him that was to come.
Adam was the only one to eat of the forbidden tree. He is the only one who transgressed the law of God. Yet all his offspring to Moses, who wrote the law of God, died. Death reigned supreme over all.
I don't have time to take this argument to the end but the point is that though God did not impute sin because he had not given a law, these men still yet fell short of the righteousness God required to have fellowship with him, which is perfect righteousness and so they all died. Why did they then die?
It is because of the consciousness of man of good and evil, right and wrong, and doing what he knows is wrong violates a man's conscience. Consider this;
Romans 2:10 But glory, honour, and peace, to every man that worketh good, to the Jew first, and also to the Gentile:
11 For there is no respect of persons with God.
12 For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law;
13 (For not the hearers of the law are just before God, but the doers of the law shall be justified.
14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another)
16 In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.
But perfect righteousness cannot come by the conscience.
He 9:8 ¶ The Holy Ghost this signifying, that the way into the holiest of all was not yet made manifest, while as the first tabernacle was yet standing:
9 Which was a figure for the time then present, in which were offered both gifts and sacrifices, that could not make him that did the service perfect, as pertaining to the conscience;
So Adam here in the text is presented as a contrasting type (figure) of Jesus Christ whom God has given to actually provide the righteousness that he requires by his perfect obedience to the law of God. When the righteousness of Christ is imputed to those of Adams race, who are under sin and death, who will receive his atonement by faith will overcome death and will have a pure conscience.
I wish I had more time to develop this thought from this text.
What do you mean by this statement? Do you mean that men have not always been saved by God's grace?
God's grace has been present and exercised in every age because God is the same in every age, he changeth not. However, He has exercised several different principles as his operative principle of divine dealing in the affairs of men. Surely you would not insist that God is still dealing with Israel under the law of Moses. If you recognize that a transition was made when Jesus died and rose again and the law of Moses ceased as the operative principle, then you are at least a little dispensational.
He 10:7 ¶ Then said I, Lo, I come (in the volume of the book it is written of me,) to do thy will, O God.
8 Above when he said, Sacrifice and offering and burnt offerings and offering for sin thou wouldest not, neither hadst pleasure therein; which are offered by the law;
9 Then said he, Lo, I come to do thy will, O God.
He taketh away the first, that he may establish the second.
10 By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
11 And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins:
If you understand that Barnabas, had not seen the grace of God in his previous 10 years in Judah as the Bible defines the grace of God simply because gentiles were not being saved, as the Bible defines salvation, until the incident between Pater and his 6 Jewish buddies were sent to the home of Cornelius for the express purpose of opening the door of faith for the first time to gentiles. Not only was it a door opening but Peter said to the Jewish brethren in Acts 11 that it was a baptism of the Holy Ghost. What is a baptism? It is an event of immersion. SO, the picture here is that the Holy Ghost is poured out on the heathen like water in such abundance that all they need do now is open their mouths and drink him in.
The Bible makes perfect sense if you are not too smart to just believe it like a little child.
BTW, I do not apologize for a thorough answer, even if it is long. It is past my bedtime so I did not proof read this. Excuse any typos or boo boos.