I don’t know if there were open theists in the seventeenth century, or if Stephen Charnock foresaw the Boyds and Pinnocks of our day, but either way, Stephen Charnock has a message for them:

   If God were changeable in his knowledge, it would make him unfit to be an object of trust to any rational creature. His revelations would want the due ground for entertainment, if his understanding were changeable; for that might be revealed as truth now which might prove false hereafter, and that as false how which hereafter might prove true; and so God would be and unfit object of obedience in regard of his precepts, and an unfit object in regard of his promises. For if he be changeable in knowledge he is defective in knowledge, and might promise that now which he would know afterwards was unfit to be promised, and, therefore, unfit to be performed. It would make him an incompetent object of dread, in regard of his threatenings; for he might threaten that now which he might know hereafter were not fit or just to be inflicted. A changeable mind and understanding cannot make a due and right judgment of things to be done, and things to be avoided; no wise man would judge it reasonable to trust a weak and flitting person. God must needs to be unchangeable in his knowledge; but as the schoolmen say, that, as the sun always shines, so God always knows; as the sun never ceaseth to shine, so God never ceaseth to know. Nothing can be hid from the vast compass of his understanding, no more than anything can shelter itself without the verge of his power.

—Stephen Charnock, The Existence and Attributes of God (Baker Books, 2005), 1:322