Good looking comment, and I partly agree. True, the sin nature cannot be removed in this life, but the new nature increases and ensures the desire for God's pleasure, which is what the goal of Christ's Cross-work--a guaranty of remaining in Him; and this is where many within Protestantism still misunderstand rebirth.
The OT believers were not under condemnation either but forgiven as we are. The difference is understanding the power of the new nature, a deeper understanding of God's mind through increased Biblical revelation of knowledge, bringing a newness of fellowship with Him, His Son and all believers. It was not given to Israel to be children of God until Jesus. They were then a people of God instead of children of God. It's just a drawing closer to God according to His sacrificial provision of the Lord Jesus.
I also only partly agree. It's true that we have a greater knowledge, since Christ has come and has introduced a new system based on the fact we already possess eternal life. But we don't have any greater spirituality is my point. It's just a completed spirituality, granting us eternal life instead of strictly the hope of eternal life.
I'm not going to try to match up with the spirituality of Enoch, Noah, David, Elijah, or John the Baptist. We are all one in Christ. Their spirituality was as deep as ours, although they did not yet possess the knowledge that they already have eternal life and can live in a system free of the encumbrances of pursuing life and not attaining it completely.
That is what the Law did--it attached many requirements to Israel's pursuit of righteousness in the search for a lasting righteousness that could not be obtained under that system. But we are not under any such system any more, since righteousness has now been achieved, through Christ, lasting duration.
I don't believe the "new nature increases and ensures the desire for God's pleasure." All of the great OT saints of old possessed the exact same zeal for God, and had the same level of spirituality and maturity as we can have now. As I said, they just didn't yet possess eternal life, and the knowledge of a righteousness that could last forever.
That could only come after Christ had come, and had delivered it to us through the process of atonement, ie dying for sin. They just didn't possess eternal life *legally.* They couldn't have that knowledge until it actually took place on the cross.
It didn't detract one bit from their spiritual virtue. It just kept them waiting for a complete picture. They were not just "people of God, and not children of God." They were both! They just didn't have full assurance yet.
It's no different today except that I would agree with you--we were not fully adopted children of God until Christ had made complete atonement for our sin. How could we be fully assured as God's children if any remnant of guilt for sin remained? The least amount of sin would keep us out of heaven!
But now that Christ has completed final atonement for his people, we can all be assured that we are children of God, and not merely hoped-to-be children of God. I believe the OT saints were children of God without legal papers that had finished going through the court of heaven. Now that those papers have been fully processed and delivered to us, we can claim not just to be children of God, but also *saved* children of God. :)