In the Old Testament, we see different aspects of the work of the Holy Spirit. In the New Testament, one of the most important works of the Holy Spirit is the work of regeneration, by which He changes the disposition of our hearts and then dwells within us, never to depart. He works within us to work out our sanctification and bring us into conformity to Christ.
In the Old Testament, you couldn’t be born again apart from the work of the Holy Spirit. The Holy Spirit did His work of regeneration then as He does now. The Holy Spirit indwelt people in the old covenant just as He does now. The Holy Spirit worked towards the sanctification of the saints in the Old Testament, just as He does now.
How, then, can we talk about the Spirit departing from Saul (1 Sam. 16:14)? Well, there was another aspect of the work of the Holy Spirit that was very important in the Old Testament, namely the charismatic endowment of power from God to enable people to perform a particular task or an office. The prophets were endowed from on high. Moses was endowed from on high. The kings, on many occasions, were endowed by this gift of the Holy Spirit to empower them to carry out their work. Where the case of Saul is concerned, the Spirit that departs from him is not the Spirit of regeneration; it’s that anointing God had given to him to fulfill his function as a king.
That raises a whole question of whether Saul was ever a Christian. I don’t think he was a believer. It was possible that God would endow or equip people like Samson and others with this divine empowerment for their vocation without at the same time saving them.
Now, I don’t think that happens in the New Testament. That’s the significance of Pentecost. We not only receive regeneration, sanctification, and indwelling, but from the day of Pentecost onward we also receive the anointing of God to carry out our task as believers in this world. In the Old Testament, that anointing was restricted to a handful of people. In the New Testament, it’s universal, to the whole body.
When God told Moses to select seventy men that he knew to be elders of Israel, God took of the Spirit that was upon Moses and anointed those seventy to help him. Joshua objected when it was reported, “Eldad and Medad are prophesying in the camp,” and Moses said, “Are you envious for my sake? Would that all of God’s people would be prophets and that He would put His Spirit upon them” (Num. 11:27–29). That was just a prayer that Moses said.
By the time you get to Joel, that prayer becomes a prophecy. Joel is saying that God would, in the future of redemptive history, pour out His Spirit not just on seventy, but on the whole community of believers (Joel 2:28–29). That’s the significance of Pentecost—God fulfills that prophecy of Joel and pours out that anointing power upon all of us.