@Ashishraj Those are great questions. If it were me, I would not focus on the fact that Christ has covered all sins already, but rather that all people who have faith in God are covered by Christ and in Christ. All of the saints throughout history from Abel to Zechariah to Paul to the last Christian left on earth are reconciled through Jesus.
But how can that be if Jesus had not yet come when Abraham was alive? Here are a few thoughts that I hope you find helpful.
I think sometimes we approach questions that are enjoyable and profitable to prayerfully ponder, but where we need to begin with obedience even without perfect understanding. There is a time where we must let God be God and come to Him as He is - I actually think, with my very limited experience, that perhaps this approach would be very logical to a Muslim? For Westerners this a huge struggle - to recognize that God’s ways are above our ways and while we certainly should pursue understanding, knowing God and understanding Him are two very different things. And we can know without fully understanding - like a child their parent.
“I do not seek to understand in order that I may believe, but rather, I believe in order that I may understand” - Anselm of Canterbury
Psalms 131:1-3 -My heart is not proud, Lord,
my eyes are not haughty;
I do not concern myself with great matters
or things too wonderful for me.
2 But I have calmed and quieted myself,
I am like a weaned child with its mother;
like a weaned child I am content.
3 Israel, put your hope in the Lord
both now and forevermore.
The Means and the Object
The OT saints put their faith in God (and Jesus is God). Therefore their faith was in the right Person. The object of their faith was correct.
However, they did not yet understand the means of salvation - how it would be accomplished. But because the object of their faith was correct, their faith was counted as righteousness.
- God is the object of faith
- the cross of Christ is the means of salvation
The OT Saints Hoped in God’s Salvation
All of the prophets looked forward to what God’s Spirit foretold about Messiah. They did not fully comprehend it, but they trusted in God and in His salvation. Their faith in God translates to faith in Christ because Christ is God’s means for salvation.
Hebrews 11:1-2 - Now faith is confidence in what we hope for and assurance about what we do not see. 2 This is what the ancients were commended for.
John 8:56 - Your father Abraham rejoiced at the thought of seeing my day; he saw it and was glad.
Luke 2:25 - At that time there was a man in Jerusalem named Simeon. He was righteous and devout and was eagerly waiting for the Messiah to come and rescue Israel.
I Peter 1:10-12 - Concerning this salvation, the prophets, who spoke of the grace that was to come to you, searched intently and with the greatest care, 11 trying to find out the time and circumstances to which the Spirit of Christ in them was pointing when he predicted the sufferings of the Messiah and the glories that would follow. 12 It was revealed to them that they were not serving themselves but you, when they spoke of the things that have now been told you by those who have preached the gospel to you by the Holy Spirit sent from heaven. Even angels long to look into these things.
To Know God is to Know Jesus / To Know Jesus is to Know God - Jesus is God
Part of this mystery is wrapped up in the fact that Jesus is God. To know one is to know the other.
We are saved not simply by believing in the death and resurrection of Jesus, but by knowing Jesus Himself. That means that anyone who truly knows God has already met Jesus and would gladly trust completely in Him. In fact, if you truly trust in the living God you have already trusted in Jesus! You have placed your faith in the Creator! Someone who knows God knows Christ.
John 8:19 - Then they asked him, “Where is your father?” “You do not know me or my Father,” Jesus replied. “If you knew me, you would know my Father also.”
John 17:3 - Now this is eternal life: that they know you, the only true God, and Jesus Christ, whom you have sent.
The OT a Shadow and a Tutor to Lead us to Christ
God had to prepare the human race to be capable as a whole of understanding the Person and work of Jesus. The Old Testament laws and sacrifices were a shadow and tutor to lead us all to Jesus. God works on the timescale of history and Christ is at the center.
Regarding sacrifices, I recommend reading the whole of Hebrews 10.
Acts 17:26-27 - From one man he made all the nations, that they should inhabit the whole earth; and he marked out their appointed times in history and the boundaries of their lands. 27 God did this so that they would seek him and perhaps reach out for him and find him, though he is not far from any one of us.
Hebrews 10:1 - The law is only a shadow of the good things that are coming—not the realities themselves.
Colossians 2:16-17 - Therefore do not let anyone judge you by what you eat or drink, or with regard to a religious festival, a New Moon celebration or a Sabbath day. 17 These are a shadow of the things that were to come; the reality, however, is found in Christ.
Galatians 3:24 - Therefore the law was our tutor to bring us to Christ, that we might be justified by faith.