So, are we saying that Warfield is wrong, assuming his quote remained as his final stance on the issue?
The quotation comes in the context of an examination of another writer's presentation. In order to understand what Warfield was saying one needs to know to what he was replying. He had just quoted Jellinghaus to the effect that "the New Testament does not (“mostly”) teach justification through faith and sanctification through faith, but
justification and sanctification through faith." Warfield's concern was with the conceptual conflation which was being made by what he calls "the mediating theology." This theology claimed that the New Testament does not make the sharp conceptual distinction which is made in Lutheran and Reformed theology. Warfield's response, then, must be seen as a concern to maintain this conceptual difference. His statement gives priority to justification. This is a just reflection of Reformed theology. Justification, adoption, sanctification, is the order of the Westminster Confession and Catechisms. He then makes a statement, which he does not go on to explain, declaring that sanctification depends on justification. "Conceptually" -- the context of the statement -- this is true. Faith appropriates Christ's righteousness for justification as a matter of priority. Faith's appropriation for sanctification depends upon this "legal" priority. Nevertheless, Warfield does insist that faith relies on Christ for sanctification, that this is the same act of faith which relies on Christ for justification, and that salvation is unitary. His view is well summarised in what follows:
For the main matter, however, Jellinghaus’ expositions of the Scriptural material are not only true, but both obvious and important. It is not exact to say that the New Testament makes no conceptional distinction between justification and sanctification. But it is true to say that it is absolutely impatient of their separation from one another, and uniformly represents them as belonging together and entering as constituent parts into the one, unitary salvation which is received by faith. The significance of Jellinghaus’ exposition of the Scriptural material is that by it it is made perfectly clear that no support from the New Testament can be obtained for separating them and representing them as two distinct benefits which may be obtained apart from each other by separate acts of faith.
With this affirmation Warfield parts company with those who are quoting him as an advocate of their justification-only gospel.