A huge amount of ink and electrons have been devoted to answering that question.
Personally, I can’t understand why this is deemed such a complex issue. It all really depends on our answer to this simple question: Were Old Testament believers believers?
If the Old Testament believers were real believers, the Holy Spirit indwelt them. No one can be born again, believe, or repent without the inward work of the Holy Spirit. And no one can stay a believer for one second without the ongoing internal work of the Holy Spirit – neither in the OT nor the NT. Without the Holy Spirit constantly in and at work in our hearts, we will immediately apostatize.
So, here are the options:
1. Old Testament “believers” were not real believers.
2. Old Testament “believers” believed by the regenerating work of the Holy Spirit but kept believing without the indwelling work of the Holy Spirit.
3. Old Testament believers, like New Testament believers, believed and kept on believing as a result of the Holy Spirit’s initial and ongoing indwelling work in their hearts.
If #1 is true, then the Bible is not true (Jn. 8:56; Heb 11).
If #2 is true, then Old Testament believers were not as depraved as we are, as they did not need the ongoing indwelling work of the Holy Spirit. (And in some ways, this debate really is a debate about the nature of human depravity in the Old Testament. Could anything less or other than the indwelling of the Holy Spirit keep a believer believing, repenting, hoping, obeying, etc?)
If #3 is true, then the question that’s left is: “In what ways did the indwelling work of the Holy Spirit differ in the Old Testament compared to the New, especially post-Pentecost?” Everyone accepts there was a difference. But what was it?
That’s a question I’ll return to in coming days (there are some difficult texts to deal with that seem to contradict #3), but in the meantime let the weight, significance, and consequences of the three options clarify our thoughts.