Legalism could be definied as any attempt to rely on self-effort (even partly) to either attain or maintain our just standing before God. In Paul’s Epistle to the Galatians he warned them sternly about such false understandings of the gospel when he asked the offenders: “After beginning with the Spirit, are you now trying to attain your goal by human effort?” (Gal.3:3). Legalism always seems to have one thing in common: it’s theology denies that Christ is completely sufficient for salvation. That some additional element of self-effort, merit or faithfulness on our part is necessary to maintain a just standing before God.
As an example, those who erroneously teach that a Christian can lose his or her salvation are, in essence, denying the sufficiency of Christ to save to the utmost. They believe their sin to be greater than Christ’s grace. But we affirm that Christ’s righteousness, which he counts toward us, is not only necessary for our salvation, but sufficient. His once for all sacrifice put away sin for all time in those He has united to Himself. His salvation also means that he not only saves at the beginning but preserves us to the end, sealing us in His perfect righteousness whose blood “reminds the covenant God” not to treat us as our sins deserve.
Any attempt to add our covenant faithfulness as part of the price of redemption after regeneration is an “attempt to attain our goal by human effort” and thus a complete misapprehension of the fullness of the gospel of Jesus Christ. We must, therefore, reject any and all attempts to maintain a judicial standing before God by any act on our part. Salvation is of the Lord.