Isn’t teaching of grace (undeserved kindness) a great apostasy?
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)
What the New Testament teaches on this subject is mixed to say the least:
James 2:24 You see that a person is considered righteous by what they do and not by faith alone.
Ephesians 2:8, 9 For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— not by works, so that no one can boast.
In the early church the biggest advocates of "once saved always saved" were the Gnostics. The church fathers on the other hand taught a Catholic or JW version of salvation that could be lost by wrong action.
What I provided was the internal evidence. Jesus came to reinforce the Law and urged more ‘violent’ or intense action. (Mathew 5:17; 11:12)
Yet much more striking is the external evidence. Teaching of Grace was very prominent in Mithraism, and Church Fathers feared that people might suspect later that Christianity borrowed from pagans, hence they blamed it on Satan (as though he is teaching truth well in advance through pagan leaders so that future generation would see no value to truth when it arrives through Jesus); hence Tertullian wrote:
"The Devil, too, baptizes some that is, his own believers and faithful followers; he promises the putting away of sins by a layer (of his own); and if my memory still serves me, Mithra there, (in the kingdom of Satan) sets his marks on the foreheads of his soldiers; celebrates also the oblation of bread, and introduces an image of a resurrection, and before a sword wreathes a crown.
What also must we say to (Satan's) limiting his chief priest to a single marriage? He, too, has his virgins; he, too, has his proficients in continence. "
– Tertullian, The Prescription Against Heretics
Google search - grace in the old testament
Jesus rebuke of the Pharisees' meticulous lawmaking and law-keeping shows they had missed the purpose of the Old Testament - shining a light on the human condition and the need for a saviour.
Religious leaders obscured this simple truth:
When you change your thinking, your action is changed
when you repeat the action, habit is created
when habit is maintained, destiny is created
and anyone can have control over his thought. But degree of control varies from person to person, hence we find sin in varying degree among people. So is the case with righteousness. There were many even Bible itself refers as blameless like Job because they had control over thought.
There is nothing as easier as changing your thought—something you can do any time anywhere without the help of an external savior. Suppose, you are tempted. See what happens if you withdraw your thought from it—temptation stops.
You say ‘Google search - grace in the old testament’
The main thread in the Old Testament does not allow for Grace in the sense you take—committing sin and making use of His Grace, but it does make room for disproportionate degree of Grace if one acts in righteousness. Curse for 3-4 generations for those hating God, and blessings for “thousand generations of those who love me and keep my commandments.” (Exodus 20:5, 6)