A common argument by dispensationalists to prove that salvation was different in the
Old Testament is the fact that the Old Testament saints were not indwelt by the Holy Spirit
as we are today.
But this is simply a doctrinal sleight of hand.
You see the Holy Spirit's role in salvation isn't necessarily based upon where he dwells
but rather who he regenerates.
The Bible says in Titus 3 5, not by works of righteousness which we have done, but according
to his mercy he saved us by the washing and regeneration and renewing of the Holy Ghost.
You see the Old Testament saints who were dead in trespasses and sin had to be quickened
like we are today.
Not only that but the disciples of Jesus Christ who were in the Old Testament did not receive
the indwelling of the Holy Spirit until John chapter 20.
Now the reason the Holy Spirit did not indwell Old Testament saints is because in the Old
Testament the occupancy of the Spirit of God was in the tabernacle later on being the temple
whereas in the New Testament the Bible says in 1 Corinthians 6 19, what know ye not that
your bodies are the temple of the Holy Ghost which is in you which ye have of God and ye
are not your own.