I found an article Regeneration precedes faith by R.C. Sproul that could imply there is no synergism at all in justification. In other words no part of the human will involved in believing? It is potentially confusing in that it puts regeneration before faith. (I can only only assume he actually believes regenerations as: calling, faith, justification and new birth so that he is actually only referring the the effectual call as preceding faith which no Calvinist would find confusing. But he seems to confuse the issue by arguing against 'faith, rebirth, justification' which as far as I know nobody has ever presented in that order from a Calvinist position.)
On the other hand, as opposed to potential confusion of Sproul's statement (which might actually intend the same as this more traditional classical Calvinist) John Owen could imply that justification is synergistic. (Actually, I know Owen puts the 'initial' work of regeneration before faith, justification and sanctification, but by identifying the human 'will' as a 'cause' of salvation according to all 'Protestant divines' he seems to be sending a message with a slightly different spin from the Sproul's terminology.)
Protestant divines, until of late, have unanimously affirmed faith to be the instrumental cause of our justification. (John Owen's Works Vol 5, Justification by Faith, Chapter three – “The use of faith in Justification”. p108)
So what's going on here? How much synergism (cooperation between God and man) is involved in the 'monergism' (the work of God without human cooperation) of classical Calvinists' in the doctrine of justification?