Were not the saved of the OT under a different covenant (with God) than those of the NT ? Doesn't the verse below signify that the Holy Spirit must "put" into their hearts, and "write" into their minds ? And without God's laws first being present, then how could faith (or any other fruits) come to life within them?
[Heb 10:16 KJV]
16 This [is] the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them;
Since Jehovah, is the same as the Lord, which is the same as the Great I am, which is the same as the Word and is the same as Jesus Christ, all believers have had access to the Lord Jesus Christ and all have heard from Him.
The OT economy, was broken into two parts - Before the Law and After the Law.
Before the Law was given, a born again believers, simply believed what Jehovah revealed to them. This was enough to show their faith in the Lord. After the Law and Sacrifices were established, there was more to keep in the way of Commandments. However, they had one thing in common, believing on what the Lord had revealed.
The OT believers from either direct revelation or the Prophets - who we know spoke directly from God. God gave them the words to speak. (Deut. 18:18; Isa. 59:21; Jer. 5:14). When Jehovah spoke in direct revelation, everyone heard Him speak but everyone did not obey His commandments. Only the Elect followed His Commandments and teachings. Therefore, they were the ones born anew and had the abiding presents of the Spirit. The same could be applied to the Prophets when they spoke. The rest of Israel were just rabble following along to see what they could get.
The Old Covenant was based on types and shadows of Christ's person and completed work, after the Levitical Laws were laid down. The New Covenant is based on Christ's blood or if you prefer, on Christ Himself. All of the types and shadows, of the OT economy, of course pointed to Christ and His redemptive work.
While we now possess a clearer image of God's work in Salvation because we have the fully revealed Word in our possession, the OT believers had many of these great Truths as well. But the one requirement of both dispensations, was to believe God. That His Word was Truth. Remember what this verse said:
John_4:24 God is a Spirit: and they that worship him must worship in spirit and truth. Two things are required for proper and acceptable worship - One must be able to commune spiritually. Thus, one must have experienced the new birth and have been made spiritually alive. (Remember, this is before the coming of the Spirit to dwell in believers). - One must worship in Truth, God's revealed Truth in the Scriptures. God does not except error, not even from His own, it must be Truth.
In Heb. 10:16 - the writer is talking about what God will do after
those days. What days? The days following Christ's second advent. God, through the Spirit will do these things to those having survived the Great Tribulation and are going into the Kingdom. Mostly Jews, the veil of Moses now lifted from their eyes, This Covenant is to the house of Israel (Heb. 8:10).
Heb 8:9 Not according to the covenant that I made with their fathers In the day that I took them by the hand to lead them forth out of the land of Egypt; For they continued not in my covenant, And I regarded them not, saith the Lord.
Heb 8:10 For this is the covenant that I will make with the house of Israel After those days, saith the Lord; I will put my laws into their mind, And on their heart also will I write them: And I will be to them a God, And they shall be to me a people:
Heb 8:11 And they shall not teach every man his fellow-citizen, And every man his brother, saying, Know the Lord: For all shall know me, From the least to the greatest of them.
Heb 8:12 For I will be merciful to their iniquities, And their sins will I remember no more.
Heb. 10:16&17 have the same exact language.