When did knowledge of Christ's sacrifice become necessary for salvation? First, this question makes two assumptions:


For a person to be saved today, they must hear the gospel of Jesus Christ, and accept the gift of His death on the cross as penance for their sin.
Before the death of Christ, the requirement for salvation was faith in God's future redemptive work (See How were people saved before ~33AD? ).  This is what saved Abraham, the thief on the cross, and anyone else who was saved up to this point.


These two premises are commonly held by many Christians, including many very close friends of mine.

So my question is, given this understanding of saving faith, when, precisely, did the requirement cease to be "faith in future redemptive work" and instead become "faith in past redemptive work?"

The easy answer would of course be "The instant Christ died," but that has unfortunate implications for anyone who died 5 minutes after Christ did but did not know of Christ's death because the obituary had not yet been published.
Pre-Christianity Salvation
The concept of &quot;salvation&quot; before Christ was not the same concept as we see today.  The idea of salvation back then was that God would come and save us from whatever affliction we were dealing with.  God brought salvation from the Egyptians;  He brought salvation from the giants that controlled Canaan; He brought salvation from lions and from being thrown in a fire.
So, the idea that &quot;salvation&quot; before Christ was brought from the hope of &quot;future redemptive work&quot; is very much invalid.  &quot;Salvation&quot; before Christ was brought from following God and obeying his commands.
For illustrative purposes:

Deuteronomy 28:1 (NIV)
If you fully obey the LORD your God and carefully follow all his commands I give you today, the LORD your God will set you high above all the nations on earth.

(a secular source, a non-secular source)
Pre-Christianity Remission of Sins
However, in the context of Christianity, if we say &quot;salvation&quot;, we mean that Christ has saved us from the bondage of sin.  When a Christian says &quot;I am saved&quot;, he means that God has forgiven his sin and that he's been freed from that bondage.  The debt has been paid; the sin is forgiven.
The concept of remission of Sins pre-Christ was that of animal sacrifice.  The sacrifice of the animal was performed for the atonement of sins. (See Leviticus 4 and Leviticus 16.)
To show that sins were forgiven:

Leviticus 16:30 (NIV)
because on this day atonement will be made for you, to cleanse you. Then, before the LORD, you will be clean from all your sins.

Once the Isrealites were scattered from the promised land, the animal sacrifices stopped and prayers from the forgiveness of sins began.
I won't go into that too deep.  But here's a post on Judaism.SE that explains why they are not required to sacrifice animals: Why don't Jews sacrifice animals anymore?
The point, though, is that the Jews (pre-Jesus and currently) are not expecting future redemption to be forgiven of sins.  They are (rightly so) expecting their sins to be forgiven at the time when they ask for forgiveness and truly repent.
The Salvation of Jesus
The sacrifice of Jesus on the cross has brought about a new type of salvation:  salvation from sins.  He has provided us a way to gain access to God directly; he has become our mediator, our savior, and our redeemer.
When people hear of Jesus, his story, his resurrection, and the forgiveness of sins that we have through his sacrifice, it is at that point that they become culpable for believing or disbelieving his sacrifice.
Prior to hearing of Jesus and learning of his sacrifice and his grace and mercy, people are still under the old ways and the old rules.  (See also What happens to people who have never heard about Jesus?)
To answer your question:
The requirement was never &quot;faith in future redemptive work&quot;.  The requirement was sacrifice of atonement and repentance.  This requirement is still around today, but the atonement was the perfect atonement made on the cross.  Prior to learning about Jesus, we must make atonement in other ways.  After learning about Jesus, we can accept his amazing gift of his sacrifice as the perfect atonement.
The burden lies on the heart, not on the timing.  It's learning about Jesus that changes the &quot;salvation&quot; (forgiveness) from atonement through animal sacrifice to atonement through messianic sacrifice.
Summary
The sacrifice of Jesus completely changed the concepts of &quot;salvation&quot; and forgiveness of sins.  His sacrifice allows people to be forgiven of their sins without the sacrifice of animals.  He has become the perfect sacrifice for us.
The requirement for forgiveness was never &quot;faith in future redemptive work&quot;.  The requirement has always been faith in God and following his laws.  (Remember, Jesus didn't come to abolish the law, but to fulfill it.)
It is the point of learning of Jesus and accepting him as the Messiah that allows us to turn from other atonement to the perfect atonement.  Once we accept Jesus, we can rest on his perfect sacrifice.