Paul made a case that men are condemned because each one goes his own way - professing to be wise, they became fools. When Satan tempted Eve, she saw the fruit could make one wise....
Therefore Gid gave them over.
Isaiah said that each has turned to his own way
The law was added later
Very true.
The Law was added very much later, in fact, and the only people under the Law were those within that covenant (Deut. 5).
But the Law does reflect God's moral law (it reflects God's nature), so I can understand people using the Ten Commandments when those are already accepted. The problem with TWOTM is that it misunderstands the Law itself.
Things like murder, adultery, theft, idolatry….all of the moral law….were not wrong because of the Ten Commandments but because they are contrary to God’s nature as revealed in the world. Even before these Ten Commandments were given to Israel the moral infractions were sins against God. As these things are wrong it is no wonder they are included in God’s covenant with Israel because God is immutable. So we can use the Law to explain, but when we use it to place people under the Law we have started working against the gospel of Christ.
To illustrate, God’s moral law is not only revealed in the Law but it is also reflected to some degree in the commandments of pagan religions. You of course have the Quran, but that’s no surprise. There are also the Ten Yamas and Ten Niyamas of Hinduism. Zoroastrians believe in sin and the expiation of sin, and a judgment day when evil is destroyed and the just will inherit eternal life with the unjust suffering an eternal punishment. Mankind has a knowledge of sin and we can speak of this in evangelism (in fact, we need to explain sin). But what men need is the gospel of Christ.
When we restrict the moral authority of God to his covenant given to a specific generation of Israel for a specific time and purpose (or reduce that even more to the Ten Commandments) then we reduce God’s law to compare with the rules of any other religion and have to sell the religion in order to convict of sin. In America it isn’t too bad yet because we can still find people who believe, to some degree, that the Ten Commandments should govern our behavior.
The Way of the Master is an interesting and can be an effective tool for evangelism. But insofar as doctrine, it is a false teaching which can easily become a stumbling block placed in the paths of others.