Unfortunately, the assumptions that you (quite usefully) stated, are where the problem lies. If you notice, you have phrased salvation as being man-dependant. In other words: "man must [hear, accept, have faith]", that means that man, in one way or another, plays a part in His own salvation, however minimal or trivial we might want to say this part is.
From Genesis to Revelation, we have God correcting this mindset, always reminding man that it is He that is sovereign, not man, and that it is God that saves, and man does not help. We confuse passages that talk about man's responsibility to believe, repent and do good as if they were talking about man's ability to believe, repent and do good.
Faith and repentance are both good works, and salvation is not by works, that no one should boast (Ephesians 2:8-9). Salvation is entirely the work of God, with no help from man. I realize that this introduces the concept of predestination and calvinism, and an enormous tangent, so I will leave most of that to another question. But in summary:
He begins the work,
Ephesians 1:4 just as He chose us in Him before the foundation of the world, that we should be holy and without blame before Him in love, 5 having predestined us to adoption as sons by Jesus Christ to Himself, according to the good pleasure of His will,
He completes the work,
Philippians 1:6 being confident of this very thing, that He who has begun a good work in you will complete it until the day of Jesus Christ;
and does not leave it unfinished,
John 6:37 All that the Father gives Me will come to Me, and the one who comes to Me I will by no means cast out.
John 6:39 This is the will of the Father who sent Me, that of all He has given Me I should lose nothing, but should raise it up at the last day.
leaving nothing to chance. (Notice that the Father does the giving first before there any comes to Christ). Christ came to save that which was lost, and He leaves the 99 that are safe to rescue the one that is lost (Luke 15:14, Matthew 18:12). We are lost, not Him. He seeks and He finds.. it is not us who seek and find. So faith is a fruit of this salvation (Galatians 5:22-23), it is not a precursor. So, if God wanted to save certain people who died 5 minutes after the crucifixion, nothing can stop Him from doing so, even their lack of knowledge.
In the Old Testament (or, more precisely, pre-crucifixion times) all men were saved by the mercy of God, manifested as faith in that mercy:
Psalm 41:4 I said, "Have mercy on me, LORD; heal me, for I have sinned against you."
Abraham knew that God would provide the necessary sacrifice, and trusted Him, Paul knew that God had provided the necessary sacrifice and trusted Him, but both were chosen by God before they trusted or believed. Jesus' sacrifice transcends time, which is why He is called in Revelation 13:8 "the Lamb slain from the foundation of the world". It is a retroactive sacrifice that applies to all recipients of God's mercy. Knowledge of Christ's sacrifice is not necessary (as a prerequisite) for salvation, but it is a necessary fruit of salvation.
So, to answer your question, "When did knowledge of Christ's sacrifice become necessary for salvation?" As a prerequisite? Never. As a fruit? As soon as you believed the Gospel. Our knowledge cannot save us, only Christ can.