God always intended to give His kingdom to the Jews. God set the Jews ahead of every other people-group in the world. Jesus told the Samaritan woman at the well that salvation is of the Jews (John 4). The Jews were the chosen people.
But God also considered the Jews "under-aged", as far as the plan of redemption goes. Israel was God's people in its infancy. The Jews might have owned the whole estate -- they were promised the kingdom of God by the prophets -- but their time had not yet come. They were no better than the special slaves of God, however much He loved them. But God wanted more for them and us.
To confirm their "spiritually under-age" status, God put them under the Law of Moses. The Law of Moses was the guardian, the trustee, the elemental principles of the age, that governed the Jews. The Jews were under-age in the eyes of God for about 1,400 years.
But then Christmas morning came. God sent His son, the Lord Jesus. He was born of the virgin Mary. He was born a Jew, born under the Law system of governance. But he redeemed those under the law -- Jews who put their trust in Him -- and as a result God adopted believers as sons. Salvation was always by grace, and always by faith, and always unto everlasting life.
But the status of the New Testament believer, and the depth of our daily experience of God's grace, is greater than any Old Testament saint. As Christ said, John the Baptist was the greatest of the prophets, but even the youngest member of the kingdom of God is greater than John. As they say in sports-talk, New Testament believers are on a whole 'nother level.
Then, because God adopted us as sons, He placed his own Spirit into our hearts. The Spirit cries out, "Abba, Father!" The Spirit of sonship. The Old Testament believers were born-again (or "circumcised of heart", to use Old Testament language), and they had the Spirit with them. What a blessing. We too are born-again, but we have the Spirit inside us! What a greater blessing!
We used to be nothing more than God's slaves. Better to be God's slaves than Satan's slaves! But now that the Savi, we become God's children. And as His children, we become God's heirs. We will inherit the kingdom. We will be welcomed in when Christ returns and sets up his throne. We will not be cast out, when Christ separates the believing sheep from the faithless goats.
Because of the redeeming work of Christ, we are no longer God's slaves. Why would you want to go backward to that status? Which is better -- to be a king's employee, or his child? Do you think of yourself as the Lord's servant, or as His affectionately-beloved kid? How do you answer that question?