Union with Christ is a central doctrine in Reformed Theology, and concerns the mystical union of the believer with Christ, by faith and by the Holy Spirit. In faith the Spirit unites us to Christ, and that union is the means by which Christ's saving work is applied to us, it is the power of regeneration, and the basis on which the earthly church can and should be united.
While there may be a sense in which all of the elect are united to Christ even before they come to faith, this Union is normally spoken about in reference to our temporal experience of God's grace: the unbelieving elect person is not yet united to Christ, but instead we are united to Christ when we are given new life, the power to have faith, and freed from sin, or in other words, saved. (Though there is a logical order, the ordo salutis, from our perspective we experience these things concurrently.)
So here we come to my question: Does Reformed Theology teach that the Old Testament saints were personally united to Christ in this same way?
Reformed theologians have traditionally taught Covenant Theology, where the various Biblical covenants, including the Old (Mosaic) and the New, are seen as aspects of the one eternal Covenant of Grace. So the Westminster Confession says:
WCF 7.6: Under the gospel, when Christ the substance was exhibited, the ordinances in which this covenant is dispensed, are the preaching of the Word, and the administration of the sacraments of Baptism and the Lord's Supper; which, though fewer in number, and administered with more simplicity and less outward glory, yet in them it is held forth in more fullness, evidence, and spiritual efficacy, to all nations, both Jews and Gentiles; and is called the New Testament. There are not, therefore, two covenants of grace differing in substance, but one and the same under various dispensations.
WCF 8.6: Although the work of redemption was not actually wrought by Christ till after his incarnation, yet the virtue, efficacy, and benefits thereof were communicated into the elect, in all ages successively from the beginning of the world, in and by those promises, types, and sacrifices wherein he was revealed, and signified to be the seed of the woman, which should bruise the serpent's head, and the Lamb slain from the beginning of the world, being yesterday and today the same and for ever.
These paragraphs would seem to indicate that yes, the OT saints were united to Christ in the same way as NT Christians are. But it's not explicit, and there are some factors which would argue against it.
First is that at Pentecost there seems to have been a fundamental change of state for the disciples whom the Holy Spirit came upon. Before that moment Jesus's disciples had faith, and the faith of the Christian is the same faith as that of Abraham (Romans 4:16). But the indwelling presence of the Spirit seems like something new; indeed Peter in Acts 2:16-21 says that the Spirit's coming upon them is the fulfilment of Joel 2:28-32, this "pouring out" of the Spirit being something new from the perspective of the OT prophets. When Paul describes the blessings of Israel in Romans 9:4-5 the Spirit is not one of them.
A second factor is that the NT consistently describes the Spirit's indwelling as permanent. Several verses describe the Spirit as our guarantee of the rest of God's blessings (2 Cor 1:22, 2 Cor 5:5, Eph 1:13-14). In contrast the OT often speaks of the Spirit departing from someone or being taken from them (Judges 16:20, 1 Sam 16:14, Ps 51:11, Is 59:21), and many times when the Spirit comes to someone (Judges 3:10, 6:34, Ezek 2:2), it comes to someone we would most naturally describe as already having faith. Now there are many ways those verses are understood, but I've often heard it said (though not necessarily by Reformed teachers) that the indwelling of saints in the OT was only temporary, instead of the permanent indwelling Christians receive.
So how does Reformed Theology understand the role of the Holy Spirit in the life of the Old Testament believer, and whether those believers should best be described as being personally united to Christ?
It is conceivable that Reformed Baptists may have a different answer to Reformed Paedobaptists as many of them reject Covenant Theology and would not say that there was only one covenant that applied equally to Old and New Testament saints. If this is the case, a good answer would explain the position of both Reformed Baptists and Paedobaptists.