Galatians 4:23
“The son of the slave wife was born in a human attempt to bring about the fulfillment of God’s promise. But the son of the freeborn wife was born as God’s own fulfillment of his promise.”
This passage is referring to Abraham and Sarah. The Lord promised them a son in their old age. While they believed God, they didn't understand how this could be. So they sought to make it happen with Abraham and Haggar. While a child was the result, God had different ideas. He wanted Sarah to be the birth mother.
The message that Paul brings is that God was to fulfill the promise. The couple had responsibilities, but only so much. Theirs was to sleep together when God restored their abilities. It was a miracle. They had long lost the ability to bear a child. The Lord restored them.
This is true with Salvation. Jesus could only accomplish this. Thousands of years showed the Jews that they couldn't keep the law. They needed the blood sacrifice to cover their sins. This was temporary, as the scripture says, the blood of bulls and goats could not take away sin (Hebrews 10:4). It's only through Jesus' sacrifice that we are made right with God.
If many were asked if God would allow them to enter heaven, they would cite their works. Common responses are "I'm not a bad person", "I've done more good than bad", "I've basically kept the commandments", "He is a god of love, He wouldn't send me to an eternity of hell for what I've done" and so on. All of these responses demonstrates reliance upon their own works. I thought the same. Many will also reference their faith: "I go to church" or "I volunteer at church".
Unless it's full reliance on God's mercy and the substitutionary sacrifice of Jesus, it's based on works. As was stated the Old Testament demonstrated the insufficiency of it. It wasn't supposed to be a complete working system. It was there to show that. It was preparation of Jesus' coming. The inefficiency of it made Jesus' coming good news.
The Galatians were missing this. Which is a shame because they had it right. Now they were persuaded by some clever talking individuals who convinced them to adopt the law along with faith as a supplement. Paul is making comparisons and points to convince them they had it right. They are to jettison their error and go back to the pure.
Many have this error. This persuasive point is the reason for this letter. Just as the Galatians fell into error, many will do so. This letter is meant to extract that error and lead them from backsliding or into salvation for the first time.
Lord, thank You for these letters which bring us into a wonderful faith that gives full reliance upon You. The error of our first parents is corrected. We get to renew that relationship with You as You intended. Thank You.