[TheRealThing]

Old Testament saints lived their lives looking forward to the coming of Christ who, through His shed blood on the cross imparted salvation to all "by grace through faith". The souls of Old Testament saints therefore were sort of in queue, until the coming of Christ who paid their sin debt with His unspeakable gift on the cross. Old Testament saints demonstrated their faith by the way they lived and observance of God's law. This had the affect of 'setting them apart, a peculiar people'.

We look backward to the crucifixion which, freed us from the penalty of the law (OT), but which law none the less, defines those of us who are saved. That statement may seem duplicitous however, look at it like this. The Holy Spirit changes the NT believer's life at conversion giving him a 'new nature' and therefore his desire to live a sinful lifestyle is forever removed. New Testament saints are therefore set apart by their new nature which, causes them to want to honor God with their lives. Spent in service and giving, part of that giving is to point folks in the right direction, calling out sin for what it truly is. This is possible by the indwelling of God's own Holy Spirit which, empowers the NT saint.

It's a matter of choice in both times. Old Testament saints chose to honor God with holy lives spent honoring the law, which was set forth in clear terms by the Priests of Levy and the Prophets. New Testament saints choose to repent of their sin before the Son of God, believing He died on the cross for their sin. We are then sealed by the Holy Spirit, new creatures in Christ. Either way, God made a propitiation for sin, before the death of Christ on the cross, and after.