Since Christ was crucified from the foundation of the world, God kept telling the Old Covenant Jews that He had redeemed them. The Lord says this through Isaiah.
But now, this is what the Lord says—he who created you, Jacob, he who formed you, Israel: “Do not fear, for I have redeemed you; I have summoned you by name; you are mine. - Isaiah 43:1
When Old Testament people trusted in God and did the right thing in His eyes, it was evidence of trusting in Christ.
The people of today don't realize that God has redeemed them through Christ. Their sins are forgiven. All they have to do is humble themselves and receive the free gift. For those who aren't able to enter heaven at the end of their lives, it won't be because God had refused them, but because they'd refused Him. All are welcome.
My dear children, I write this to you so that you will not sin. But if anybody does sin, we have an advocate with the Father—Jesus Christ, the Righteous One. 2 He is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world. - 1John 2:1-2