Legalism is technically a living by laws and rules in order to progress. This is an absolute MUST in secular society! Imagine a nation without law! However, legalism is not for the Church.
Legalism was only used twice. Once was in the time
before Adam fell into sin, and then secondly during Old Mosaic Covenant in the Bible. Adam was told to not eat the forbidden fruit. GRACE was not involved in that at all, and since Adam was not sinful, he did not require grace. But when Mosaic law arrived on the scene, God used a legalistic system with people who were sinful, and it could not work. And God knew that. But the New Testament says that God did it this way in order to instruct mankind as to how sinful sin really is, by having man realize he cannot keep rules and thereby serve God without grace. So, God was preparing Israel for the coming of Grace with the fullness of Christ.
Legalism is involved in Christianity where this concept of Grace is not fully understood. In the church, legalism is a no-no. Living by rules implies we do not need the strength of God and His divine empowerment, but only the mere information of what is right and wrong in order for us to do the right and avoid the wrong. Legalism has no faith in the fact that God actually empowers believers by His Spirit to live the resultant life that Law and its legalism tried to accomplish. (See
1 Tim 1:5, 14 compare with
Rom 8:3-4 &
Gal 5:23).
Legalism is basically self making self progress in serving God without the empowerment of the Spirit, whereas Christian liberty does not condone sin any more than Law did, but it accomplishes the results Law could not accomplish in having us successfully serve God by believing God to actually empower us by His Spirit to live righteously.