I understand that being sealed with the Spirit is something distinct to our dispensation of the church. I also understand that Old Testament Saints were not sealed by the Spirit and that the Spirit could come and go, sometimes for the purpose of only empowering people for a certain task and then he would leave. The Spirit was never promised to a believer. In fact you could be a believer and not even be indwelt by God’s Spirit. My question is why you don’t believe in Eternal Security for the Old Testament Saints?
Eternal Security depends on the Holy Spirit being sealed within us guaranteeing our inheritance. Before 2 Cor. 1:21-22 this was never mentioned. But His seal was promised to the Church there, and in 2 Cor. 5:5, Ephes 1:13-14, and Ephes. 4:30.
Throughout the Old Testament the Holy Spirit was with people and would come upon them in power from time to time but was never sealed within them. In fact He was taken away from Saul (1 Sam. 16:14), and David feared the same thing would happen to him after his sin with Bathsheba (Psalm 51:11). Such a thing can never happen to us.