Good example from scripture. I also like the following:
Tell me, you who want to be under the Law, do you not understand what the Law says? For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. His son by the slave woman was born according to the flesh, but his son by the free woman was born because of the promise. These things serve as illustrations, for the women represent two covenants. One covenant is from Mount Sinai and bears children into slavery: This is Hagar. Now Hagar stands for Mount Sinai in Arabia and corresponds to the present
-day Jerusalem, because she is in slavery with her children. But the Jerusalem above is free, and she is our
mother.
For it is written:“Rejoice, O barren woman,
who bears no children;
break forth and cry aloud,
you who have never travailed;
because more are the children of the desolate woman,
than of her who has a husband.”
Now you, brothers, like Isaac, are children of the promise. At that time, however, the son born by the flesh
persecuted the son born by the Spirit. It is the same now. But what does the Scripture say? “Expel the slave woman and her son, for the slave woman’s son will never share in the inheritance with the free woman’s son.”
Therefore, brothers, we are not children of the slave woman, but of the free woman."
It appears that there are always going to be those people who-- no matter how many illustrations the Holy Spirit has used--they just will not let go of the works of the law as a requirement for salvation. They say "Oh no, it is through Christ only that we have salvation" while at the same time presenting a list of works that must be done along side of what Jesus did, such as the keeping of the ten commandments, keeping the Sabbath, abstaining from certain foods, etc.. They never come to the understand that we are no longer under that covenant. In fact, we were never under that covenant. Gentiles have put themselves under the works of the law. The only reason for adding these works would be to gain favor with God (Lord, I kept the Sabbath and the ten commandments) and the Lord will say "woopdeedoo! I shed my blood for you and paid the penalty for your sins! Depart from me I never knew you!" No wonder the scripture says "Many will say to me on that day Lord, Lord."
"May I never boast except in the cross of our Lord Jesus Christ, through which the world has been crucified to me, and I to the world."