Was the Holy Spirit working in the lives of believers in the Old Testament? There is no doubt that the inauguration of the New Covenant (Jer. 31:31-34; Ezek. 11:19-21; 36:25-27; 37:14, 24-28; Joel 2:28-29) and the birth of the church in Acts 2, brought in a new era of the Holy Spirit's work in the lives of believers. The Holy Spirit began to permanently indwell those who placed their faith in Jesus Christ (Rom. 8:9-11; I Cor. 12:13; Eph. 4:30).
It is also clear that the Holy Spirit came upon certain Old Testament believers and even unbelievers to empower them for certain tasks. Balaam and Saul are examples of two unbelievers that the Holy Spirit empowered temporarily. Balaam was empowered by the Holy Spirit to prophesy (Numb. 24:2-9) and Saul was empowered by the Holy Spirit to prophesy and serve as king (I Sam. 10:9-11; 11:6; 16:14). But apart from temporary empowerment did the Holy Spirit have anything to do with Old Testament believers? The answer is yes, but the Scriptural support is thin.
The Holy Spirit seems to have offered some ongoing assistance to Old Testament believers, but we don't know to what extent. Jesus, speaking of the coming of the Holy Spirit to permanently indwell believers as promised in the New Covenant, said to His disciples:
I will ask the Father, and He will give you another Helper, that He may be with you forever; that is the Spirit of truth, whom the world cannot receive, because it does not see Him or know Him, but you know Him because He abides with you and will be in you. (Jn. 14:16–17)
Notice that Jesus said the Holy Spirit was already abiding "with" the disciples, but at some future point would be "in" them. This tells us that the Holy Spirit was involved in the lives of believers in the Old Testament era. To what extent, we don’t know exactly. We also know the Holy Spirit was involved in saving people in the Old Testament.
In John 3 Jesus speaks to Nicodemus, a leader of the Pharisees. Being a leader of the Pharisees Nicodemus would have been an expert in the Scriptures. Jesus spoke to Nicodemus in the Old Testament era, before the birth of the church, before the inauguration of the New Covenant, while still under the Law of Moses. Jesus spoke of being born again by the Holy Spirit and Nicodemus was having difficulty following Jesus, not understanding what Jesus meant. Jesus then said:
Are you the teacher of Israel and do not understand these things? (Jn. 3:10)
Jesus made it clear that people in the Old Testament era were born again by the Holy Spirit or they couldn't enter the kingdom of God (vs. 5). He said Nicodemus, being a leader of the Pharisees, should have known about the Holy Spirit's regenerating work in the Old Testament era. This tells us that the Holy Spirit, in both the Old Testament and New Testament eras, was and still is involved in causing people to be born again. I wish Jesus would have said a little bit more as to why or where in the Old Testament Scriptures Nicodemus could have learned this, but He didn't.
Yet we learn from Jesus two interesting truths about the Holy Spirit in the Old Testament. First, the Holy Spirit has always been involved in the process of saving sinners, causing them to be born again from Adam until today. Secondly, the Holy Spirit was "with" Old Testament believers to assist them in living for God's glory. After Jesus inaugurated the New Covenant with His death, the Holy Spirit now permanently indwells, empowers, and assists believers in greater ways to walk according to God’s Word.