This article is from Denver Seminary, where Dr. Blomberg and Dr. Groothuis both teach.

Excerpt:

Middle knowledge is a proposed solution to predestination vs. free will, to divine sovereignty and human responsibility, going all the way back to the medieval Jesuit priest Molina (so sometimes it’s also called Molinism).

Classic Calvinists, properly concerned to safeguard divine sovereignty, have typically rejected any theological system that bases God’s predestining activity on the basis merely of his foreknowledge of how humans will respond to the gospel, because they’re convinced that makes human free choice the ultimate determiner. Romans 8:29, of course, doesbase predestination on God’s foreknowledge, but the Calvinist typically argues that the Greek prōginoskō (“foreknow”) there begins already to shade over into the idea of election because in the Old Testament the Hebrew yādā‘ (“know”) often appears roughly synonymous with “choose.” That would explain why Paul doesn’t say just that those whom God foreknew he also predestined, which could be seen as tautologous, but “predestined to be conformed to the image of his Son.”

Classic Arminians and Wesleyans, properly concerned to safeguard human freedom and accountability, have typically rejected any theological system that bases God’s predestining activity on the basis merely of his gratuitous election, because they’re convinced that makes human free choice ultimately a chimera. They often point out that prōginoskō is not the same verb as just ginoskō (which the LXX uses to translate yādā‘ and that in Greek it most commonly means simple knowledge in advance. Thus predestination is based on God’s foreknowledge.

Middle knowledge argues for both! If open theism in recent years has diminished divine omniscience more than orthodoxy has classically permitted, middle knowledge magnifies or expands God’s omniscience beyond what most people have thought about. But it makes good sense: middle knowledge claims that God’s perfect, infinite knowledge must be able to know not only what sentient creatures will freely choose in all situations in their lives but what everyone would do in every possible situation that they could confront. Even more magnificently, divine and unlimited knowledge must be able to discern what all possibly created beings would do in all possible situations (or, as philosophers like to say, all possible worlds).

So far so good, I hope. Now here’s the rub. Because there will only ever have been a finite number of humans created before God brings this world as we know it to an end, that means there remain countless uncreated beings that he could have chosen to create but didn’t. So God’s very choice to create you and me and not various other people he could have is an act of his sovereign election utterly prior to our existence. Calvinists should be happy. But it is based on knowing what we will and would do in all actual and all possible situations. Arminians should be happy. Thus, William Lane Craig in The Only Wise God defends this view from a libertarian Arminian perspective; Alvin Plantinga in a chapel talk at Denver Seminary years ago did the same from a libertarian Calvinist perspective, and Terrance Tiessen in Providence and Prayer does so from a compatibilist Calvinist perspective.

Somebody in the comments mentions that even Paul Helm has written positively about middle knowledge. Middle Knowledge shows what is going on under the hood of classical Calvinism, which affirms both the sovereignty of God and the responsibility of man. Middle knowledge explains how God can achieve both goals simultaneously.

The apostle Paul has this to say in Acts 17:22-31:

22 So Paul stood in the midst of the Areopagus and said, “Men of Athens, I observe that you are very religious in all respects.
23 “For while I was passing through and examining the objects of your worship, I also found an altar with this inscription, ‘ N D ‘ Therefore what you worship in ignorance, this I proclaim to you.
24 “The God who made the world and all things in it, since He is Lord of heaven and earth, does not dwell in temples made with hands;
25 nor is He served by human hands, as though He needed anything, since He Himself gives to all people life and breath and all things;
26 and He made from one man every nation of mankind to live on all the face of the earth, having determined their appointed times and the boundaries of their habitation,
27 that they would seek God, if perhaps they might grope for Him and find Him, though He is not far from each one of us;
28 for in Him we live and move and exist, as even some of your own poets have said, ‘For we also are His children.’
29 “Being then the children of God, we ought not to think that the Divine Nature is like gold or silver or stone, an image formed by the art and thought of man.
30 “Therefore having overlooked the times of ignorance, God is now declaring to men that all people everywhere should repent,
31 because He has fixed a day in which He will judge the world in righteousness through a Man whom He has appointed, having furnished proof to all men by raising Him from the dead.”

In this research paper, Craig explains in detail what middle knowledge is, and how it works to resolve problems, like “what about those who have never heard of Jesus?” and “how can we reconcile divine sovereignty and human freedom and responsibility?” Highly recommended tool for your tool chest.