William Lane Craig lists an answer to Open Theist’s claims that Calvinists rely on dignum deo over the Bible:
1. Openists have their own conception of what is dignum deo, and they don’t hesitate to draw on it when the Scriptures are silent. For example, if the openists are right that the Bible doesn’t clearly teach exhaustive omniscience with respect to the future, it’s no less true that it doesn’t clearly teach exhaustive omniscience with respect to the past and present; yet openists accept the latter. Why? Presumably because ignorance of any detail of the past and present would not be dignum deo.
The main problem with this as an answer to the Open Theist’s objection is that it really does not answer the objection at all. Instead, the argument is “well, you too.” There is a formal name for the fallacy known as the Tu quoque fallacy. Wikipedia sums the fallacy up nicely: “To clarify, although the person being attacked might indeed be acting inconsistently or hypocritically, such behavior does not invalidate the position presented.”
If two criminals were talking, one might say to the other: “You are a thief, you need to repent.”
The second might respond: “You are a thief too”.
Notice though that the second point does not answer the first. Thieves need to repent, regardless as to who says it.
William Lane Craig offers his remarks as the only answer to the Openness objection to Dignum Deo. It would be fine if Craig offered up compelling reasons for his belief and then added that Openness advocates were hypocrites, but focusing on the hypocrisy rather than the point is avoiding the real issues. In fact, some Open Theists do “accept the later” and by Craig avoiding the point, he successfully avoids answering a legitimate objection raised by consistent Open Theists.