The basis for this charge is because although God takes the first step in seeking, convicting, knocking and opening hearts to receive Him, “this step is not decisive, and can be thwarted by the sinner. If the sinner refuses to cooperate with or assent to this proffered grace, then grace is to no avail.” (What is Reformed Theology, p.187)
So in other words, any theology whereby God sovereignly gives a person the power of contrary choice, must be Pelagianism by definition. That is the argument.
Sproul continues: “The problem is this: If grace is necessary but not effectual, what makes it work? ... Why does one sinner respond to the offer of grace positively and the other negatively?” (What is Reformed Theology, p.187)
In other words, if one person receives Christ, but another does not, then what makes the difference? (To the Calvinist, God is the decisive difference, illuming one but not the other.)
Sproul writes: “Does grace assist the sinner in cooperating with grace, or does the sinner cooperate by the power of the flesh alone? If the latter, it is unvarnished Pelagianism. If the former, it is still Pelagianism in that grace merely facilitates regeneration and salvation.” (What is Reformed Theology, pp.187-188)
This gets back to the original point, so here comes the key argument for Sproul:
Sproul states: “If God merely offers to change my heart, what will that accomplish for me as long as my heart remains opposed to him? If he offers me grace while I am a slave to sin and still in the flesh, what good is the offer? Saving grace does not merely offer regeneration, it regenerates. This is what makes grace so gracious: God unilaterally and monergistically does for us what we cannot do for ourselves.” (What is Reformed Theology, p.188)
Why would Sproul think that Prevenient Grace is just an “offer” rather than some actual operation taking place at the heart level? Acts 16:14, concerning Lydia, states that God opened her heart to respond to the Gospel. Note that it does not say that her old stony heart was swapped out for a new heart of flesh. (Calvinists commonly read that into the passage.) Why shouldn’t we understand the passage simply to mean what it says, in that God enabled her to believe? Why shouldn’t we conclude the same thing about others who hear the Gospel, that is, that God gets down at the heart level, convicts them of their sin, and enables them to believe, though not being decisive, that is, a determinant, but instead forces the person into a one-way-or-the-other choice? Is God sovereignly entitled to do so? The question seems absurd, but is also quite realistic, since it is often insisted that for God to give someone such a choice, namely, the power of contrary choice, would be tantamount to God giving away His sovereignty.
Sproul summarizes: “What the unregenerate person desperately needs in order to come to faith is regeneration.” (What is Reformed Theology, p.188)
Why? If you say, “Man is so depraved that God must....” then all that you have done is limited God. Think of it this way: There are at least three types of God-can’ts. There is the illogical can’t, which is really not true inability, but rather a logic puzzle, such as whether God can make a square circle, or create a rock so big that even He cannot lift. There is also the type of God-can’t where God can’t sin, which is not true inability, in that God lacks capacity to sin, but instead is a God-can’t due to preference, in that God’s preference is not to sin, due to His nature. Finally, there is the God-can’t which is true inability, in that God simply lacks power to do something, and this is precisely what many allege, if it is maintained that God lacks the divine power and capacity to sovereignly deliver a person the power of contrary choice. I'd rather that Calvinists insist that God uses Irresistible Grace, rather than Prevenient Grace, out of preference rather than a “lack of other options.”