Let me jump right into the middle and start splashing about in your criticism (which is all well taken—indeed appreciated!).
1) The relationship between the old covenant and the new covenant is complex and I do not have all the intricacies worked out yet. The work of the Holy Spirit before Pentecost is part of this problem.
My present understanding is that the benefits promised on the basis of the new covenant have been enjoyed by regenerate people in all ages. These include justification by faith, the forgiveness of sins (which Paul equates in Romans 4:6, 7) and the possession of the Holy Spirit.
I am keenly aware of the problems in handling John’s view of the Spirit and some of the texts in Hebrews (see below). 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).
2) Stephen makes a very suggestive comment to the Jews of the post-Pentecost period: “You hard necked and uncircumcised in heart and ears, you are always resisting the Holy Spirit, as your fathers, so also you” (Acts 7:51). Does this not mean that Stephen and Luke thought of generation after generation of Jews resisting the Holy Spirit just as the Jews of the post-Pentecost era did?
3) Paul picks up the motif of the “uncircumcised heart” and says that (true) circumcision is of the heart, just like being a (true) Jew is a secret matter of the heart (Romans 2:28ff). In other words, these terms correspond to the “new creature” or “new man”; they mark off the truly new person who is in saving fellowship with God.
But I would argue that Paul is simply repeating a truth that the Old Testament made plain. In Deuteronomy 10:16 Moses appeals, like Stephen, “Circumcise then your heart and stiffen your neck no more.” What in reality was he calling for? Or hear Jeremiah 4:4, “Remove the foreskin of your hearts!” Or Ezekiel 18:31, “Cast away from you all your transgressions which you have committed and make yourselves a new heart and a new spirit!”
I think each of these Old Testament texts and others like them are calling for a present realization of the promise of the new covenant when “I will give you a new heart and put a new spirit within you and I will remove the heart of stone and give you a heart of flesh. And I will put my Spirit within you and cause you to walk in my statutes” (Ezekiel 36:26-28). That Jeremiah and Ezekiel and Moses predicted this great redemptive work for the future when they spoke to the nation as a whole does not mean that the individual could not experience it already.
4) Moses predicts a coming day when “the Lord will circumcise your heart and the heart of your descendants to love the Lord your God with all your heart and with all your soul that you may live” (Deuteronomy 30:6). Similarly, Jeremiah (31:31-33; 32:39-42) and Ezekiel (36:24-27; 11:19, 20) forecast such a day. Nevertheless all three (Deuteronomy 10:16; Jeremiah 4:4; Ezekiel 18:31) call for the people to experience in their hearts already what is predicted in large measure for the nation in the future.
And, in fact, the depiction of Old Testament saints is a depiction of men who do enjoy the blessings of the new covenant. Under the new covenant God will “put his law within them and write it on their heart” (Jeremiah 31:33). But Isaiah 51:7 says, “Listen to me you who know righteousness, a people in whose heart is my law; do not fear the reproach of men.” And is not the entirety of Psalm 119 a prayer for the present realization of this same promise?
5) 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.
6) In Jeremiah 32:39 God promises, “I will give them one heart and one way that they may fear me always for their own good.” But in Psalm 86:11 David prays, “Teach me thy way, O Lord, that I may walk in thy truth; make my heart one to fear thy name.” He prayed for and experienced the hoped-for blessing.
7) Dozens of other examples could be given of how the saints of the Old Testament enjoyed the spiritual blessings of the new covenant, and I see no serious problem here since there is no reason to think that the promise of spiritual renewal made to the nation could not already be experienced by some individuals.
As Paul says in Romans 11:7 and 9:8, only the elect in Israel were truly “children of God”; the rest were hardened. That is, under the old covenant what Moses said in Deuteronomy 29:4 was true of most Jews: “The Lord has not given you a heart to know, nor eyes to see, nor ears to hear.” But to some he did give such a heart so that they loved him with all their heart (Deuteronomy 30:6) and delighted in his law to do it. If this is not the regenerate heart, then we don’t need regenerate hearts.
8) Your statement that regeneration begins at Pentecost is to me theologically impossible unless we are just confusing terms. 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.
9) But now, if I understand you correctly, you think that the array of exemplary saints in Hebrews 11, with no regenerative work of God in their heart, by some natural human power overcame their enmity toward God and took such great delight in his promises that they sacrificed even their lives to remain faithful.
If this is possible, I ask again: who needs regeneration? If a man can have forgiveness of sins (Psalm 32), obedience of faith (Hebrews 11), intimate fellowship with God (Psalm 23), joy (Psalm 43:4; 51:8, 12) and peace (Psalm 119:165; 4:7, 8) before the Spirit of God ever comes to him, then his coming has lost all its significance and the idea of “regeneration” is reduced to I know not what.
10) You are probably thinking now of all the texts in the New Testament that say something unique or new is happening when Christ comes. Something new has come. Christ has come. Once men trusted in the God whose promise was grounded in his mighty deeds in history and in his word. Now men trust in the God who raised Jesus from the dead (1 Peter 1:21) and set their hope on the grace coming to them at his revelation (1 Peter 1:13).
The new things that have happened are, at least, 1) that the good news of God’s saving reign is overflowing the banks of Israel and his Spirit is being poured out on all flesh, 2) the knowledge of the triune God is now fuller so that our fellowship with him can accord with better knowledge, and 3) the ground of redemption is now understandable as we look to the cross and resurrection and our assurance shifts its focus to these great finished events.
To use a metaphor by way of summary, the saving rule of God, which has always been in effect, was inaugurated at the coming of Jesus. The point of the metaphor is this: a man can fulfill all the main functions of an office before he is openly inaugurated. Dr. David Hubbard did the work of the president of Fuller Theological Seminary several months before he was openly inaugurated with the sound of rushing wind and tongues of fire and people from many lands each hearing a great oration. The incarnation and Pentecost are the inaugural festivities of God’s saving reign.
11) A word about the two problem texts you mention. I think this idea of inauguration fits well with Hebrews 9:15 and its reference to “transgressions committed under the first covenant.” The death of Jesus procured the forgiveness which all the elect under the old covenant enjoyed. Verses 16 and 17 are more problematic. As Michel (p. 317) points out, a new Wortgleichnis has come in here because “covenant” in the Old Testament sense now becomes “last will and testament.” All the language is juridical.
Since the writer introduces this metaphor, which is foreign to the Old Testament use of berith (Hebrew for "covenant"), we cannot press this application of it, I think, so that the “new covenant” is limited to the meaning of “testament.” So the statement that a diatheke (Greek for "covenant") “is not yet ischuei while the testator lives” cannot be used against my view because the covenant I am talking about does not have the limitation of a “last will and testament.”
The point of the passage seems to be that in so far as the covenant Christ inaugurates can be compared to a will, it is most assuredly now in force since the testator has died.
12) The hardest verses for me are John 14:16ff, “And I will ask the Father and he will give you another helper, that he may be with you forever, the Spirit of truth, whom the world cannot receive because it does not behold him or know him, but you know him because he abides with you and will be in you.”
Also 7:39 is especially troublesome: “the Spirit was not yet given because Jesus was not yet glorified.” I’m not at all sure I understand John’s pneumatology (especially 20:22), but I suspect the key to it is found in the identification of the earthly Jesus with the Holy Spirit: “He is with you but will be in you” (14:17 cf. 6:63).
Since the Spirit which Christians enjoy is known to be the Spirit of Christ (Romans 8:9), perhaps John thinks it inappropriate to think of the Spirit in this sense as having come. Theologically I would ask who the agent is in accomplishing the “drawing” of 6:44 and the enabling of 6:65? Do we postulate that God the Father works directly without the agency of the Spirit? Or can we not suppose that the Spirit was redemptively at work during Christ’s earthly ministry and that the Spirit which has not yet come is that particular manifestation of the Spirit which will equip the apostles uniquely for calling to remembrance all things (14:26) and guiding into all truth (16:13)?
These are just gropings. I do not have the problem of Johannine pneumatology solved.