I never have had a Calvinist of any level of education or experience come forward and offer an explanation why effectual grace cannot be resisted but sanctifiying grace can be resisted. As I’ve argued in the past ( http://www.ovrlnd.com/theology/arminianthought.html ):
1) Within Calvinist theology, common grace is given to all with the goal of sustaining the good, but never with salvation in mind. Only effectual grace has the goal of justification in view. Hence within Calvinist theology there are differing kinds of grace. Therefore to the Arminian mind, the Calvinist apologist must make a case from the Bible that differing kinds of grace do indeed abound and the Arminian apologist needs to set forth that there is but one kind of grace that operates within differing modes.
2) Within Arminian thought, there is only one kind of grace which is said to go before, or, operate ahead of, (hence the term “prevenient”) that enables all good and righteous acts and thoughts, thereby providing to our Lord all the glory of such acts and thoughts—for it is His doing. Ultimately within Arminians thought, prevenient grace has the goal of sanctification but its more immediate goal is justification and these two different goals can be thought of as simply modes of operation by the same kind of grace that always goes before encouraging and enabling. Within the Arminian system of thought, prevenient grace is said to be offered to all. In this sense prevenient grace is irresistible because the offer of grace to all cannot be denied. Nevertheless because prevenient grace has justification and sanctification in view and not all are justified and neither is sanctification ever complete or perfect even for the most holiest of saints, prevenient grace is also said to be resistible in both its justification and sanctification modes of operation. Positively, in acceptance, prevenient grace is said to be passive and negatively, in rejection, the heart is said to be active—a volitional stand against God and the willful suppression of truth (cf. Paul’s argument in Romans 1). Consequently, arguments against prevenient grace that hold as a premise that the human will as the basis of salvation, are simply in error—although they are very common. So common that I’ll state it again: arguments against freewill are not addressing Arminian theology. The only people who give a hoot about freewill are underinformed lay persons, philosophers, and Calvinist apologists who believe they are addressing Arminian soteriology in a meaningful way when they talk about free will but are, in fact, not.
3) In Calvinist thought, one must be regenerated before being able to exercise faith. This prompts a question: is regeneration by grace and is this grace not “going before”? Remember prevenient simply means “going before.”
Furthermore, as stated above, to the Arminian mind there is an inner ambiguity within Calvinist thought that says, in effect, there are different kinds of grace. Some kinds of grace are resistible and some kinds are not. This is clearly illustrated when we consider that the elect may not resist saving grace, but for some reason they can resist sanctifying grace. Herein we have two different kinds of grace within the same individual. As an additional note: in all the Calvinist systematics I have read, I have never heard of a Reformed thinker argue that even common grace is resistible. This is one area I would like to confirm from those who have conducted similar surveys because if true, if the Calvinist scheme of grace is never resistible, howbeit that sanctification is resistible? I’m wide open for folks to share and teach me.
Ultimately, Arminian thinkers hold that Calvinist apologists need to first build an argument of differing kinds of grace from the discipline of biblical theology before assuming differing kinds of grace that operate differently depending upon the situation and they need to do this as an a priori before basing other arguments upon it—especially so when Calvinist apologists seek to prove other points supported by this premise to Arminians who will, in turn, reject these arguments because of the unproven premise.
In contrast to Calvinist thought, in Arminian thought grace is said to always be going before. In point of fact, grace is said to always operate preveniently: it is going before we are justified drawing and empowering our faith. Following justification, grace continues to move preveniently, preparing the way for “righteous responses” to and with our acts of faith, generating elements and fore-tastes of God’s perfection within us. When we “improve” upon grace, it moves on ahead, preveniently improving us still further toward the glory which our Lord has for us. In every act of grace, our Lord’s action is first, our faith is responsitory.
Therefore, in so far as grace is concerned, Arminian thought has a unifying principle as to how grace always operates.