The Old Testament gives us many examples of what it would take to earn eternity. In fact, part of the focus of the OT is about how God’s people were trying to be right with God on their own efforts. They tried and failed over and over again. Well, some tried at least.
One of the best success stories would be David. He was considered to be a man after God’s own heart. He even penned Psalm 15 that tells of the kind of man who would be right with God.
“He who walks blamelessly and does what is right
and speaks truth in his heart;
who does not slander with his tongue
and does no evil to his neighbor,
nor takes up a reproach against his friend” (Psalm 15:2-3)
Now remember David (a man after God’s own heart) is saying these things. He is also the man who had an affair with one of his soldier’s wife, got her pregnant and had the husband killed so he could take her as his own wife in an attempt to cover up the affair. I would say that was evil to his neighbor. I would say that was taking up reproach against a friend.
You see, David was actually right that only the blameless will inherit eternity and be united with God. But he was wrong in considering himself (or any mortal) blameless. They would continually repent and ask for forgiveness, sacrifice lambs and pray and they believed this cleansed them and made them right with God again. But the debt we owed for our own sins could never be paid by the blood of a lamb. It could only be paid by the blood of The Lamb.
This is why Jesus came, lived, taught, died and rose again. Because there is no amount of good deeds that will ever qualify us as blameless. Only the blood of Jesus can wash away our sins and restore us to blameless.
When man sets the bar, we always make it reachable on our own merit. But God sets that bar much higher and we can never reach it without Christ Jesus.