The next inquiry is what constitutes the atonement
The answer to this inquiry has been already, in part, unavoidably anticipated. Under this head I will show:
1. That Christ's obedience to the moral law as a covenant of works, did not constitute the atonement.
(1.) Christ owed obedience to the moral law, both as God and man.
He was under as much obligation to be perfectly benevolent as any moral agent is. It was, therefore, impossible for Him to perform any works of supererogation; that is, so far as obedience to law was concerned, He could, neither as God nor as man, do anything more than fulfil its obligations.
(2.) Had He obeyed for us, He would not have suffered for us. Were His obedience to be substituted for our obedience, He need not certainly have both fulfilled the law for us, as our substitute, under a covenant of works, and at the same time have suffered as a substitute, in submitting to the penalty of the law.
(3.) If He obeyed the law as our substitute, then why should our own return to personal obedience be insisted upon as a sine qua non of our salvation?
(4.) The idea that any part of the atonement consisted in Christ's obeying the law for us, and in our stead and behalf, represents God as requiring:
(a.) The obedience of our substitute.
(b.) The same suffering, as if no obedience had been rendered.
(c.) Our repentance.
(d.) Our return to personal obedience.
(e.) And then represents him as, after all, ascribing our salvation to grace. Strange grace this, that requires a debt to be paid several times over, before the obligation is discharged!
—Finney's Systematic Theology