First, let’s try to understand the meaning of Revelation 13:8 which says, “And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the Lamb SLAIN FROM THE FOUNDATION OF THE WORLD.”  I think we might all realize that the Lord Jesus was crucified at a particular time while on the earth.  The year that Jesus is traditionally believed to have died is thought to be around 30 to 33 AD.  If that is true, then how could He have been “slain from the foundation of the world”?  This really means that the Lord Jesus was slain in the Father’s eternal counsels even before He created the world.  We read in 1 Peter 1:18-20, “Forasmuch as ye know that ye were not redeemed with corruptible things, as silver and gold, from your vain conversation received by tradition from your fathers; But with the precious blood of Christ, as of a lamb without blemish and without spot: Who verily was FOREORDAINED BEFORE THE FOUNDATION OF THE WORLD, but was manifest in these last times for you.”  I hope this helps you understand Revelation 13:8 concerning the time the Lord Jesus was slain for our sins. 

Were the Old Testament saints saved by the death of Christ?  I will immediately say that anyone who has ever been saved, or will ever be saved, will be saved by the death of Christ on the cross when He gave Himself as the only acceptable sacrifice to God for the payment of our sin debt.  The Lord Jesus emphatically stated in John 14:6, “…I am the way, the truth, and the life: no man cometh unto the Father, but by me.”

We know that the sacrifices and offerings used under the Law in the Old Testament could never put away even one sin.  Hebrews 10:1-4, 11 tells us, “For the law having a shadow of good things to come, and not the very image of the things, can never with those sacrifices which they offered year by year continually make the comers thereunto perfect. For then would they not have ceased to be offered? because that the worshippers once purged should have had no more conscience of sins. But in those sacrifices there is a remembrance again made of sins every year. For it is not possible that the blood of bulls and of goats should take away sins…And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins.”  Since these things could not put away sin, why did the Lord command His people to make these sacrifices? Let’s go back to Hebrews 9:9 where we read of these sacrifices.  That says, “Which was a figure for the time then present, in which were offered both gifts and sacrifices, that could not make him that did the service perfect, as pertaining to the conscience.”  These sacrifices were a reminder to those of the awfulness of their sins before God. 

Through the offerings of these sacrifices by those who obeyed the Lord, there was ATONEMENT made.  We read in Exodus 30:10, “And Aaron shall make an ATONEMENT upon the horns of it once in a year with the blood of the sin offering of atonements: once in the year shall he make ATONEMENT upon it throughout your generations: it is most holy unto the LORD.”  To atone simply means “to cover.”  These sacrifices could never put away sins, but the proper use could atone or cover up those sins until something could be done to PUT AWAY their sins.  That ‘something’ was the offering of Christ for all our sins. “And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance.” (Hebrews 9:15 – NKJV).

So, in the giving of Himself, the Lord Jesus paid the sin debt of all those who believe on Him.  In His death, the Lord redeemed those who were under the “first covenant” or those who followed Him under the rituals of the Law.  The Apostle Paul wrote in Romans 4:3, “For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.”  This means Abraham had FAITH in God and His promises to him.  Those who had faith in God and in the knowledge that He gave of Himself to them, were saved by the shed blood of the Lord Jesus Christ.  (CC)  (689.6)