Upon believing in Christ does the standard of the Law change? Is the acceptable standard of obedience before the Law still and only perfection? On what basis then are the "good works" of believers acceptable as such before God?
Neonomians allege, that though we cannot fulfil that perfect obedience which the law of works demanded, yet God has been graciously pleased, for Christ’s sake, to give us a new law, according to which, sincere obedience, or faith, repentance, and sincere obedience, are accepted as our justifying righteousness. It may be here remarked, that the Scripture nowhere gives the slightest intimation that a near and milder law has been substituted in place of the law of works originally given to man. Christ came "not to destroy the law, but to fulfil it." The gospel was never designed to teach sinners that God will now accept of a sincere instead of a perfect obedience, but to direct them to Jesus Christ as "the end of the law for righteousness to every one that believeth." The idea of a new law, adapted to the present condition of human nature, reflects the greatest dishonour both upon the law and the Lawgiver; for it assumes that the Lawgiver is mutable, and that the law first given to man demanded too much.
IX. Of Justification - Robert Shaw, The Reformed Faith: An Exposition of the Westminster Confession of Faith (1845)
So, on what basis are the believer's "good works of sanctification" then deemed acceptable by God?
In short, I affirm, that not by our own merit but by faith alone, are both our persons and works justified; and that the justification of works depends on the justification of the person, as the effect on the cause. (John Calvin, Acts of the Council of Trent with the Antidote)
Yes, believers seek to obey, seek to be faithful, and seek to walk in a manner worthy of their Lord. Yet the biblical truth is that their Obedience/Sanctification, indeed their entire Salvation, is accomplished only by God's grace through faith alone in Christ alone (Eph. 2: 8-9) that no man may boast.
whereas Christ saith plainly - When ye have done all that are commanded to you, say, 'We are unprofitable servants' (39 Articles, WCF, Luke 17:10).