The crux of the matter of impeccability comes down to the same thing as in the matter of the inability of sinners. BOTH are fully answered by the principle that there is a difference between a moral inability and a natural inability. A natural inability is one in which the man is unable to do something no matter how much (hypothetically) he might want to do it. In other words, the man’s will has no effect either way on a natural inability. But a moral inability is one in which the man is unable to do something precisely because the moral state of his heart will not allow him to want to do it. In other words, the man’s will is really all that is involved in a moral inability.
As a Baptist centrist, I agree with Andrew Fuller’s view of the inability of sinners. In the ordinary use of the term, ability, as viewed with disregard to will, it’s meaning is simple and clear–one is either able or unable. But in the theological use of the term, as mingled with the idea of will, the utmost care must be used in understanding the meaning. Both Calvinists and Arminians, when speaking of total depravity, confuse the ordinary use with the theological use, and lead to the absurdity of men not being able to come to Christ no matter how much they might want to come to Him. But when Scripture speaks of such inability, it speaks of it in only a moral sense. Fuller (The Gospel Worthy of All Acceptation) explains it this way:
…It is just as impossible, no doubt, for any person to do that which he has no mind to do, as to perform that which surpasses his natural powers; and hence it is that the same terms are used in the one case as in the other. Those who were under the dominion of envy and malignity “could not speak peaceably;” and those who have “eyes full of adultery cannot cease from sin.” Hence, also, the following language, “How can ye, being evil, speak good things?”
In Gen. 37:4, it is said that Joseph’s brothers “could not speak peaceably unto him.” Here a moral inability is spoken of. Why is it that they could not speak peaceably unto him? Or, is it even true that they could not? Since Scripture says it this way, then it must be true… but in what way? You see, when the ideas of will and ability are wrapped up together, then the meaning gains a complexity and nuance that the natural, ordinary use does not have. It is true that they were unable to speak peaceably, but only because their hateful hearts refused to speak peaceably to him. It is not true in the simple, ordinary sense of the word, that they could not speak peaceably no matter how much they might want to. That is the difference between a volitional freedom and a moral freedom. Volitionally, there was nothing to stop Joseph’s brothers from speaking peaceably except their own willful refusal; but in the moral sense, their sinful hatred made their continuing refusal certain. Their inability was not some lack of natural powers to do what was right, but only an aversion of heart that left them without excuse.
There are some other examples of this expression. But it is commonly understood, anyway—take your taxes for example. If you hate what the government is doing with your money, and you tell them that you are angry and unable to pay your taxes, they will still put you in jail. Why is that? If you are unable, then you are unable. The problem is that they will immediately understand the implied distinction between an inability that consists in want of natural powers and an inability that consists only in want of will, and they will arrest you for having no excuse.
This distinction between moral and natural inability also applies to the impeccability of Christ. He was morally unable to sin because the moral state of His heart refused to sin and made certain His continual refusal; but He was NATURALLY ABLE to choose sin just as any other man is naturally able. It was not true that He could not have sinned no matter how much (hypothetically speaking) He might have wanted to; but rather, He simply did not want to, and the moral state of His heart made it certain that He would never want to.
Ken Hamrick, 2013