God's word tells us what sin is;
1 John 3
[4] Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.
So we know that if a person sins they are breaking a law of God.
The scriptures also tells us that Adam sinned, thus Adam broke a law of God;
Rom.5
[14] Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come.
Now that we know Adam broke a law of God, let's read what effect did that transgression have upon ALL of mankind;
1 Cor.15
[22] For as in Adam all die, even so in Christ shall all be made alive.
Rom.5
[12] Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
God's word tells us that ALL have sinned, thus death was passed upon ALL men.
Now just how inclusive is "...all die...", "...so death passed upon all men..." and "...for that all have sinned:"? That would be ALL of mankind. From the first man Adam up until now...ALL have sinned and death was passed to ALL of mankind. That would include David, Abraham and ALL of the saints of the bible. And what death was it that was passed to ALL of mankind? Let's read.....
Heb.9
[27] And as it is appointed unto men once to die, but after this the judgment:
Paul tells us that ALL men were appointed to die a natural death. So even Adam was never to keep a flesh and blood body even if he had not broken God's law. But after this natural death Paul then speaks of...."...the judgement." Let's find if there is another death. The one that was passed on to ALL man;
Rev.2
[11] He that hath an ear, let him hear what the Spirit saith unto the churches; He that overcometh shall not be hurt of the second death.
Rev.20
[6] Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.
So now we see what death was passed on to all of mankind. The 'second'death! The death that will not allow one eternal life with God. This is the death that Adam brought upon ALL of mankind for breaking just one of God's laws.
Now as we know...Abraham, David and ALL of the other saints died with a good report. They were ALL righteous in the sight of the Lord. But what was yet needed for salvation? What act HAD to be accomplished before ALL of their sins were wiped away? Let's read...
Heb.9
[11] But Christ being come an high priest of good things to come, by a greater and more perfect tabernacle, not made with hands, that is to say, not of this building;
[12] Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption for us.
[15] And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance.
So Christ had to shed His blood for any sins that were done in the OT so that those that were called might receive salvation. And what about those that died AFTER Christ shed His blood? Let's continue...
[28] So Christ was once offered to bear the sins of many; and unto them that look for him shall he appear the second time without sin unto salvation.
So the shedding of Christ blood wiped away the sins of those that were in the OT and even those AFTER His death look for Him in the future.
Now let's confirm...
Heb.10
[10] By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
[12] But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God;
[14] For by one offering he hath perfected for ever them that are sanctified.
So...the blood of Christ removed the sins of ALL that are set apart......past AND future.
What could have saved all of mankind other than the shedding of the blood of Christ?
Nothing!
.