What is legalism? A basic idea is that it is following the letter of the Law and not the Spirit. An example might be driving no faster than 25mph in a 25mph zone. The Spirit of the Law would be driving as fast as you want to in a 25 mph zone as long as you were safe.
In Matthew Ch 5 Jesus emphasizes the Spirit of the Law. The Pharisees were exploiting “loopholes” in the Law. An example would be eye for an eye tooth for a tooth. If one had struck out in violence the victim then had a right to do the same. Jesus clarified this point by saying to turn the other cheek. To not repay evil with evil 5:38-41.
In verse 18 he says that not one dot or tittle of the Law will pass away. 19 doing away with the least commandment will merit someone being called least in the Kingdom of Heaven. 20 says that we must keep the law beyond that of the pharisees. These scriptures clearly point that we must keep the Letter of the Law.
Note that Jesus did not say that the Law wasn’t to be followed anymore, but he came to fulfill it (5:17). Nor did he say just to obey the spirit. If that were the case we don’t need to obey traffic laws as long as people are safe. Following just the Letter of the Law, it would be fine to run people over as long as we went the speed limit. But rather we must follow both. We must drive under the speed limit, obeying the Letter, while making sure we keep others safe, the Spirit.
Often we hear in the Church of Americanity that we just need to keep the Spirit of the Law and were ok. Do we need to follow the Spirit of the Law? What about the Letter? No, if we are born again, we have grace, through Jesus Christ and have no direct consequences. Should we, absolutely not! Paul clearly says that we should by no means continue sinning (Rom 6). If we love God, wouldn’t we want to keep His commandments? We have grace to fall back on when we stumble, but it is not an excuse to stumble.
Jesus concludes chapter 5 by this challenging statement
(Esv) Mat 5:48 You therefore must be perfect, as your heavenly Father is perfect.