Three issues. Justification, forgiveness, and the Holy Spirit…
Justification: was it by works or faith?
Forgiveness of sins: was it through law or grace?
The Holy Spirit: always indwelling or only after Pentecost?
I don’t claim to have it all worked out, but I look at Scripture and have a hard time reconciling these three as some other who make a bigger distinction between these two covenants.
Justification: Its always been by faith.
Abraham was justified by faith. The OT saints saved by looking forward to a coming messiah and we look back on the risen Savior Jesus. I don’t know how you can read Rom. 4:17-21 any other way.
Forgiveness of Sins…not a new concept apart from sacrifice.
Jeremiah 31:34 says that under the new covenant “I will forgive their iniquity, and their sin I will remember no more.” But David prays in Psalm 25:7, “Do not remember the sins of my youth or my transgressions.” And he exults (with words that, according to Paul, teach justification by faith, Romans 4:6-8); “How blessed is he whose transgression is forgiven, whose sin is covered. How blessed is the man to whom the Lord does not impute iniquity” (Psalm 32:1ff). “As far as the east is from the west, so far has he removed our transgressions from us (already!)” (Psalm 103:12). The Old Testament saints enjoyed the promised blessing of forgiveness. (John Piper, Re: The New Covenant and the Holy Spirit in the Old Testament). But that is another that many wouldn’t necessarily argue about, but the Holy Spirit is questionable. We read texts about the spirit that hasn’t come yet because Jesus hadn’t yet ben glorified (John 7:39), Jesus tells us of the Spirit who will come after him as a helper (John 14:15-24), and the coming the the spirit in Acts 2. With all this can you still say OT saints possessed the spirit?
The Holy Spirit: yep, he has been around for long time.
The Holy Spirit has been possessed by all people of the church (all believers throughout all time). John Piper says that “Pentecost does not bother me as much because I don’t think Luke thought in terms of the Holy Spirit’s entering into the human situation for the first time at 9:00 a.m. Pentecost morning. Nor is Pentecost presented as the point where people are regenerated or saved. It is the open inauguration of a new and wider (“upon all flesh”) outpouring of God’s Spirit to empower witnessing: “You shall receive power when the Holy Spirit has come upon you and you shall be my witnesses…” (Acts 1:8). He goes on to say that it is theologically impossible. Must a man be born again to enter the Kingdom? Why? Because until a man is born of the Spirit he is simply flesh (John 3:6), and mere flesh cannot enter the Kingdom of God (1 Corinthians 15:50). Until a man is born of the Spirit and led by the Spirit he is dead (Ephesians 2:1-2). For those who are according to the flesh set their minds on the things of the flesh, but those who are according to the Spirit set their minds on the things of the Spirit. For the mind set on the flesh is death, but the mind set on the Spirit is life and peace; because the mind set on the flesh is hostile toward God, for it does not subject itself to the law of God’s, for it cannot (Romans 8:4-7). Paul’s kata sarka and kata pneuma of Romans 8:5 are essentially the same as John’s born of sarx and born of pneuma in John 3:6. Both agree that without the Spirit’s renewing work a man cannot enter eternal life for he is hostile to God and cannot subject himself to God’s law. (John Piper, Re: The New Covenant and the Holy Spirit in the Old Testament).