Remember, Abraham is given as the example "of all who are of faith" and he is pre-Mosaic. The Mosaic Covenant did not change salvation by grace through faith (Acts 10:43) previous to Moses or UNDER Moses or AFTER Moses as the same way, same gospel, same savior and same justification by faith has been since Genesis 3:15 to Revelation.
Remember, the law was "added" but not in order to save anyone but only to reveal the knowledge of sin and lead them to faith in Christ which was pictured in all the sacrifices.
In terms of Justification, OT sacrifices could not take away sins.
Christ went and preached to the spirits in Sheol (1Pet 3:19, Psalm 16:10), so that now, "...at this time [God might declare] his righteousness: that he might be just, and the justifier of him which believeth in Jesus" (Rom. 3:26), because "[He wanted] to declare his righteousness for the remission of sins that are past, through [His forbearance]" (Rom. 3:25), as "it is not possible that the blood of bulls and of goats should take away sins" (Heb. 10:4).
- The OT saints were in those, as Ecclesiastes calls it, "days of darkness" (Eccles. 11:8) until Christ went into Sheol (Psalm 16:10) and brought them out of it.
Yet Leviticus 4:20 tells us that by that OT sin offering practice, the priest could make atonement for them, and [the sin] was forgiven them.
I have a suggestion for this, and that is that Christ is a High priest (Psalm 110:4).
Just like when Christ took some conflicting ideas in scriptures and made harmony between them by showing that those figures in scripture were actually showing it's type (Matt. 12:1-8), so I present the same.
- The High priest's practice in Leviticus 4:20 is a type of Christ (Heb. 3:1, 8:3-6, 10:1-10).
- This is why it says that the High Priest's practice could make an atonement for them (Lev. 4:20).
In terms of salvation:
Numbers 15:32-36 doesn't sound like salvation to me...
(Whether or not this man was one of the elect of Heb. 9:15 is a question I ponder).
It seems to me that righteousness was by observing the law in the old dispensation (Deut. 6:25, Luke 1:6).
"...it shall be our righteousness, if we observe to do all these commandments before the LORD our God, as he hath commanded us" (Deut. 6:25).
"...they were both righteous before God, walking in all the commandments and ordinances of the Lord blameless" (Luke 1:6).
But that things have changed now under this new dispensation of grace (Eph. 3:2, Rom. 9:30-32, 10:5-6, Gal. 4:4-5), and righteousness is now by the faith that is
now manifested (Rom. 3:21).
"But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets" (Rom. 3:21).
Let me know if you find any scriptures that conflict with this.