Ga 3:2 This only would I learn of you, Received ye the Spirit by the works of the law, or by the hearing of faith? Ga 3:6 ¶ Even as Abraham believed God, and it was accounted to him for righteousness
The word “legalism” (sometimes used in a negative light) is not in the bible, the concept simply refers to keeping the law of God, Mt 5:17. Jesus summarized the Law of God as Love for God and neighbor. The Law teaches a person that a sacrifice is necessary to obtain forgiveness of sins, Rm 10:4. Christians have to confess sin to the sacrifice (Jesus), for forgiveness. In this way the law us leads to Christ, Gal 3:24. Christ is the end or goal of the (Mosaic) law. Ro 10:4 For Christ is the end of the law unto righteousness to every one that believeth
Abraham believed (had faith in) God and was considered righteous - he did the right thing, 1 Jn 3:7. Doing the “works of the Law” is not the right way to be made acceptable to God, even though the sacrifices had to be made for a person to be forgiven after they sinned, Heb 10:8. Ga 5:4 Ye are severed from Christ, ye would be justified by the law; ye are fallen away from grace.
The first time something is done it becomes a precedent. Since Abraham originally exhibited “faith” and God accepted this, it can be said to be a law. The right way to be accepted by God was to have faith, Gen 15:6.
Ga 3:10 For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them.
If a person tries to be acceptable to God by performing the “works mandated by the Law” but does not have faith, they have broken with precedent or broken the law and are cursed on account of their disobedience.
Ro 9:30 What shall we say then? That the Gentiles, who followed not after righteousness, attained to righteousness, even the righteousness which is of faith: Ro 9:31 but Israel, following after a law of righteousness, did not arrive at that law. Ro 9:32 Wherefore? Because they sought it not by faith, but as it were by works. They stumbled at the stone of stumbling;
Heb 10:8 Saying above, Sacrifices and offerings and whole burnt offerings and sacrifices for sin thou wouldest not, neither hadst pleasure therein (the which are offered according to the law), (ASV)
Heb 10:10 By the which will we are sanctified through the offering of the body of Jesus Christ once for all.