Legalism vs. Obedience

Legalists may appear to be righteous and spiritual, but legalism ultimately fails to accomplish God’s purposes because it is based on outward performance instead of an inward change.
The legalist isolates the law from the God who gave the law. He is not so much seeking to obey God or honor Christ as he is trying obey rules that are devoid of any personal relationship, in the hope that it will earn him enough favor to go to heaven.
The letter and the spirit

"...clearly you are an epistle of Christ, ministered by us, written not with ink but by the Spirit of the living God, not on tablets of stone but on tablets of flesh, that is, of the heart. And we have such trust through Christ toward God. Not that we are sufficient of ourselves to think of anything as being from ourselves, but our sufficiency is from God, who also made us sufficient as ministers of the new covenant, not of the letter but of the Spirit; for the letter kills, but the Spirit gives life. 2 Corinthians 3:3-6 NKJV