B.B. Warfield explains (with thanks to Carl Trueman for reminding us all of this superb statement).
It belongs to the very essence of the type of Christianity propagated by the Reformation that the believer should feel himself continuously unworthy of the grace by which he lives. At the center of this type of Christianity lies the contrast of sin and grace; and about this center everything else revolves. This is in large part the meaning of the emphasis put in this type of Christianity on justification by faith. It is its conviction that there is nothing in us or done by us, at any stage of our earthly development, because of which we are acceptable to God. We must always be accepted for Christ’s sake, or we cannot ever be accepted at all. This is not true of us only “when we believe.” It is just as true after we have believed. It will continue to be true as long as we live. Our need of Christ does not cease with our believing; nor does the nature of our relation to Him or to God through Him ever alter, no matter what our attainments in Christian graces or our achievements in Christian behavior may be. It is always on His “blood and righteousness” alone that we can rest. There is never anything that we are or have or do that can take His place, or that can take a place along with Him. We are always unworthy, and all that we have or do of good is always of pure grace. Though blessed with every spiritual blessing in the heavenlies in Christ, we are still in ourselves just “miserable sinners”: “miserable sinners” saved by grace to be sure, but “miserable sinners” still, deserving in ourselves nothing but everlasting wrath. That is the attitude which the Reformers took, and that is the attitude which the Protestant world has learned from the Reformers to take, toward the relation of believers to Christ