This affirmation is about as convincing as buying a cheap Rolex at a car boot sale. The word 'Rolex' isn't enough to inspire confidence that the time piece is the genuine article. Open theists redefine omniscience and then affirm the redefinition. But if you look closely you can see how the magic trick works.
The following quotations from the late Clark Pinnock are taken from his chapter “From Augustine to Arminius: A Pilgrimage in Theology.” He frames his journey, throughout that chapter, in the language of being freed from Calvinist logic. In his new found emancipation he was now able to listen to what the Bible was saying.
A careful reading of the whole chapter, and from the extracts below, reveal, however, that there is a commitment on his part to following the logic of a non-negotiable premise, namely libertarian free will:
Finally I had to rethink the divine omniscience and reluctantly ask whether we ought to think of it as an exhaustive foreknowledge of everything that will ever happen, as even most Arminians do.
I found I could not shake off the intuition that such a total omniscience would necessarily mean that everything we will ever choose in the future will have been spelled out in the divine knowledge register, and consequently the belief that we have truly significant choices to make would seem to be mistaken.
I knew the Calvinist argument that exhaustive foreknowledge was tantamount to predestination because it implies the fixity of all things from"eternity past," and I could not shake off its logical force. I feared that, if we view God as timeless and omniscient, we will land back in the camp of theological determinism where these notions naturally belong.
It makes no sense to espouse conditionality and then threaten it by other assumptions that we make. (Clark Pinnock, “From Augustine to Arminius: A Pilgrimage in Theology” in Pinnock [ed.], The Grace of God and the Will of Man, p. 25)The same pursuit of consistency, even at the cost of revising divine omniscience can be found in author Richard Rice:
In the earlier part of this discussion we noticed the considerable difficulties encountered by those who seek to reconcile the concept of absolute divine foreknowledge with an affirmation of creaturely freedom. Now we can identify the basic cause of these problems. They arise from the attempt to combine contradictory elements from different views of God, specifically from the attempt to incorporate elements of the Calvinist view of God with the Arminian model.
The concept of absolute foreknowledge retained from Calvinism is incompatible with the dynamic portrait of God that is basic to Arminianism. Absolute foreknowledge--the idea that God sees the entire future in advance--is incompatible with the concept that God interacts with his creatures on a momentary basis.
But we cannot make such changes in our concept of God coherently while clinging to the traditional concept of divine foreknowledge. To be consistent, we must reformulate our understanding of omniscience. (Richard Rice "Divine Knowledge and Free-Will Theism" in Pinnock [ed.], The Grace of God and the Will of Man,p. 133-4)Compare the position adopted by Pinnock and Rice with the description of the Socinian denial of God's exhaustive foreknowledge found in William Cunningham's Historical Theology Volume 2 (1862).
Summarizing the Socinian argument Cunningham wrote:
That they may seem, indeed, not to derogate from God's omniscience, they admit indeed that God knows all things that are knowable; but then they contend that future contingent events, such as the future actions of responsible agents, are not knowable,--do not come within the scope of what may be known, even by an infinite Being; and, upon this ground, they allege that it is no derogation from the omniscience of God, that He does not, and cannot, know what is not knowable. (Historical Theology Volume 2, p. 173)