Hi octoberose,
Looking at Hebrews 10, this is talking to believers and trying to encourage them, explaining the change from the old covenant to the new covenant. Take notice to the first verse.
Heb 10:1 For since the law has but a shadow of the good things to come instead of the true form of these realities, it can never, by the same sacrifices that are continually offered every year, make perfect those who draw near.
So here in this chapter it is speaking of the old covenant and how the sacrifices were needed repeatedly and "can never... make perfect those" and were only "a shadow of the good things to come" later through Christ, as it states a few verses down.
Heb 10:9 then He added, "Behold, I have come to do Your will." He does away with the first (old covenant) in order to establish the second (new covenant,Christ, which it states in next verse).
v. 10 And by that will we have been sanctified through the offering of the body of Jesus Christ once for all.
Heb 10:16 "This is the (new) covenant that I will make with them AFTER those days, declares the Lord: I will put My laws on their hearts, and write them on their minds,"
Chapter 11 gives all those examples of the Faith that God did give to those in the OT and that absolutely counts for something, as it says in 1st Corinthians. Anybody's works that prove to be of righteousness, they will receive "a reward."
1Cor 3:13 each one's work will become manifest, for the Day will disclose it, because it will be revealed by fire, and the fire will test what sort of work each one has done.
v. 14 If the work that anyone has built on the foundation survives, he will receive a reward.
So when you get over to Hebrews 12 it is encouraging the believers to follow the great many witnesses/examples of faith just mentioned in chapter 11 and not to be overcome by their trials.
Heb 12:1 Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us,
This seems to be saying that surely there was never a more encouraging and persuasive faith, for the believers as the witnesses of the Patriarchs in the OT writings that was just shown in the previous chapter. This was to motive them, as they could see what the Holy Spirit had done in the OT, how much more so would it do for those who know the Christ they spoke of that was to come!
Heb 12:2 looking to Jesus, the founder and PERFECTER of our faith, who for the joy that was set before Him endured the cross, despising the shame, and is seated at the right hand of the throne of God.
And what had it just said about those in the OT...
Heb 11:39 And all these, though commended through their faith, did not receive what was promised,
v. 40 since God had provided something better for us, that apart from us they should not be made perfect.
Those of the OT must receive the gospel of Christ, repent and accept His sacrifice, you can not put salvation before Christ actually came into the world and accomplished it by His sacrifice.
Heb 12:24 and to Jesus, the mediator of a new covenant, and to the sprinkled blood that speaks a better word than the blood (sacrifices) of Abel.
Heb 9:15 Therefore he is the mediator of a new covenant, so that those who are called may receive the promised eternal (eonian) inheritance, since a death has occurred that redeems them from the transgressions committed under the first covenant.
Anyway hope this can help a bit.
mercy, peace and love
Kat