Open Theism is a strange heresy that argues while God knows all things "that can be known", the future cannot be known, and so God does not know it. This strange attempt to answer Calvinism argues that there are times throughout Scripture when God appears not to know something. He asks questions, he says certain things "never entered His heart", He even has the capability of repentance. This, Open Theists argue, means God's omniscience does not extend to the future.
The problem we run into is that God also asks questions about the past and present. In Genesis 3, we see God ask no less than 4 questions:
1. "And the Lord God called unto Adam, and said unto him, Where art thou?" (Genesis 3:9).
2. "And he said, Who told thee that thou wast naked?
3. Hast thou eaten of the tree, whereof I commanded thee that thou shouldest not eat?" (Genesis 3:11).
4. "And the Lord God said unto the woman, What is this that thou hast done? And the woman said, The serpent beguiled me, and I did eat." (Genesis 3:13).
Note, not only does God ask questions about the past and the present, He also responds to the answers. "Where are you?" is a question about the present. "Who told you you're naked?" "Have you eaten from the tree?" and "What have you done?" are all questions about the past. And as each answer is given, He turns to the last accused.
Using the logic of Open Theism, this shows God is incapable of keeping track of just two people. Could such an incompetent god really be trusted to judge us justly? Why can't we just lie to Him, like so many criminals lie to the significantly less capable human judges?
As it turns out, someone else actually tried that with God just one chapter later. "And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him. And the Lord said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper? And he said, What hast thou done? the voice of thy brother's blood crieth unto me from the ground. And now art thou cursed from the earth, which hath opened her mouth to receive thy brother's blood from thy hand; When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth." (Genesis 4:8-12).
Note, God uses the same strategy of opening with a question. "Hey Cain, where's your brother?" "I don't know", Cain lies. But God asked a question He knew the answer to. Then, God asks another question, not even giving Cain a chance to lie a second time before letting Cain know He knows Abel is dead, and Cain is the culprit.
From this, we see that God is quite capable of asking questions He knows the answer to. This is actually an effective strategy even for humans. You don't just make a statement and expect people to believe it, you walk them through the reasoning process by asking them questions and letting their answers lead them to the right conclusion. Furthermore, in court, lawyers are even trained not to ask questions they don't know the answer to! Do you think God is incapable of using the same strategy?
Throughout Scripture, we see a God who knows all things. He sees you when you're sleeping, He knows when you're awake, He knows if you've been bad or good, read His book, for goodness sake! What can the Psalmist possibly mean by "For there is not a word in my tongue, but, lo, O Lord, thou knowest it altogether" (Psalm 139:4), or "Thine eyes did see my substance, yet being unperfect; and in thy book all my members were written, which in continuance were fashioned, when as yet there was none of them" (v16)? How can God say to Jeremiah "Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee, and I ordained thee a prophet unto the nations" (Jeremiah 1:5)? What sense is there in God telling Isaiah "Declaring the end from the beginning, and from ancient times the things that are not yet done, saying, My counsel shall stand, and I will do all my pleasure:" (Isaiah 46:10)?
All of these things, and more, are in the Bible precisely because God does know all things. He knows the end from the beginning, including His prophets, including the words on our tongues, including all the days of our lives, and including His plans and purposes. If God does not know the future, we cannot be sure He holds ours, but as He knows all things, as His word explicitly tells us, we have no need to fear His incompetence, for He has none. Open Theists promote the strangest of all heresies, for even unbelievers, not knowing our Bible, know our God knows all. Let us therefore set aside such childish heresies and let God be God.