“The Law through Moses was given, grace and truth through Jesus Christ became” (John 1:17). I translate it this way because the verbs used here are significant.
The Law “was given” through Moses; Moses didn’t give the Law, it was given through him; the verb is in the passive. Moses did what he could only do, he was merely the vehicle chosen by God for transmission of the Law to us in the form that it was given to him. That’s all Moses could do.
Grace and truth, on the other hand, “became” through Jesus Christ; they weren’t given, they became. The verb used here is ginomai and its use almost always signifies a supernatural happening as in “the word became flesh”. Grace and truth could only become through Jesus Christ for when the Word became flesh and dwelt amongst us, He was “full of grace and truth”.
In another sense, Moses could only command righteousness; but Jesus, on the other hand, produced righteousness and will reproduce it in us if we will allow Him. Moses also could only forbid us to sin; but Jesus, on the other hand, will set us free from sin, both its power and its presence, if we will allow Him. Moses could not save anyone, nor can any man; but Jesus, on the other hand, if He is enshrined in the heart of the believer as Lord, is also Saviour, saving us from the penalty of sin and offering us the eternal life that is found in knowing the one true God and Jesus Christ His Son.