In the Old Testament the basic teaching is that no sin would go unpunished. Suddenly, it takes an opposite turn in the New Testament where grace is shown as flowing through Jesus to sinners who put faith in the sacrificial death of Jesus. Interestingly, New Testament writers admit that both the teachings are in opposition to each other. (Galatians 5:4)
Here some difficult questions arise. What was the need for such an opposite turn for God? Even if it would produce good result, would a God of integrity opt for it? But in this case, after the teaching of Grace was introduced, it has no effect on the habit of people’s sinning—even religious leaders themselves are caught for various heinous crimes such as child-abuse. That would mean it is a change of teaching for the benefit of religious leaders because they flouted all the possibilities of Grace and acted in OT style when it came to donation to the church (which means teaching of Grace is to attract people to church, not for the leaders)—Acts 5:1-11
Such great opposite turn is not even alluded to in the great prophecy about Jesus (Deuteronomy 18:18, 19) Jesus too has no idea about a teaching of Grace which is seen till his last words. (Revelation 22:12) Teaching of Grace cannot originate from God because He knows that those who want to do right will always do right even when there is no law given (eg. Joseph resisted temptress when there was no law) and those who want to do wrong will always do wrong even when there is law given (eg. David’s murder and adultery)—especially so God has already declared that grace is ineffective (Isaiah 26:10)