Regarding OT believers, they were also justified by grace thru faith.
Abraham believed God and it was accounted to him for righteousness..before he offered up his son Isaac.
Job believed in a redeemer.
And Jesus, referring to the OT.. said search the scriptures, in them think ye have eternal life.
OT believers were saved, but in promise only. Abraham was credited with righteousness, but it still had to happen (Romans 3:25-26). OT believers who heard and believed the Gospel before the cross, were given the right to become Sons of God, born again (John 1:12-13), but not until Pentecost when they were placed into Christ Jesus with that Promise of the Father, the Holy Spirit indwelling them.
Many people think that the NT begins at Mathew chapter one, but that's still the OT. Matthew Mark, Luke and John are all still the OT up until each records the cross. That why we still see the Levitical system and the Priesthood in place in the Gospels. Many errors, even whole denominations were birthed from that error. Pentecostals and Charismatics and many reformed people both conclude that if the Gospels are the NT, then what happened in Acts must be something more. They both conclude this same idea of more power. While that is true, that power comes from being born again, which OT believers didn't yet have until they were placed into Christ Jesus withe the baptism by/with the Holy Spirit. Strange bedfellows, theologically speaking, but true. The NT begins with the first believers placed into Christ Jesus, the birth of the Church. This baptism with the Holy Spirit baptized them into Christ Jesus. It was owed to them. That's what made them born again and justified, being "placed into" Jesus, the One True Church. The OT heroes died having not received that Promise (Hebrews 11:39-40). They were kept in Abrahams bosom in Hades until Jesus was lifted up (John 3:13-15).
Everything waited for the death, resurrection, and ascension of Jesus, as He said, He must first be glorified, lifted up, then He will send the Holy Spirit. That's what birthed the NT, the indwelling of the Holy Spirit that placed the first believers in Christ.
Hades, simply means realm of the spirits. Since the NT was translated into Greek, Hades was the word that best described Sheol from the OT. The idea of Hades comes from Sheol in the OT, not Greek mythology. So, OT Sheol is translated as Hades in the NT. The KJV erroneously translated that same word as 'Hell' in many places. or, as some have noted, that the idea of hell back then was more in line with what we know of Hades today. Anyways, this was one of the errors that the NKJV fixed, or updated.
Here's a link that everyone should have in their library that touched on this and more. I found this about twenty years ago and it was saying many of the things that I was discovering at the same time.
BIBLE STUDY MANUALS: EPHESIANS 4:7-9 HE LED THE CAPTIVES FREE
Galatians 3: 22-29 But the Scripture has confined all under sin, that the promise by faith in Jesus Christ might be given to those who believe. But before faith came, we were kept under guard by the law, kept for the faith which would afterward be revealed. Therefore the law was our tutor to bring us to Christ, that we might be justified by faith. But after faith has come, we are no longer under a tutor. For you are all sons of God through faith in Christ Jesus. For as many of you as were baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. And if you are Christ's, then you are Abraham's seed, and heirs according to the promise.