Open theism, also known as openness theology, is a theological movement that has developed within Christianity as a rejection of the synthesis of Greek philosophy and Christian theology. It is a version of free will theism and arises out of the free will theistic tradition of the church, which goes back to the early Church Fathers. Open theism is typically advanced as a biblically motivated and logically consistent theology of human and divine freedom (in the libertarian sense), with an emphasis on what this means for the content of God's foreknowledge and exercise of God's power.

Open theist theologian Thomas Jay Oord identifies four paths to open and relational theology:

Roger E. Olson said that open theism triggered the "most significant controversy about the doctrine of God in evangelical thought" in the late 20th and early 21st centuries.

In short, open theism posits that since God and humans are free, God's knowledge is dynamic and God's providence flexible. Whereas several versions of traditional theism picture God's knowledge of the future as a singular, fixed trajectory, open theism sees it as a plurality of branching possibilities, with some possibilities becoming settled as time moves forward. Thus, the future, as well as God's knowledge of it, is open (hence, "open" theism). Other versions of classical theism hold that God fully determines the future, entailing that there is no free choice (the future is closed). Yet other versions of classical theism hold that, though there is freedom of choice, God's omniscience necessitates God's foreknowing what free choices are made (God's foreknowledge is closed). Open theists hold that these versions of classical theism do not agree with the biblical concept of God; the biblical understanding of divine and creaturely freedom; and/or result in incoherence. Open theists tend to emphasize that God's most fundamental character trait is love and that this trait is unchangeable. They also (in contrast to traditional theism) tend to hold that the biblical portrait is of a God deeply moved by creation, experiencing a variety of feelings in response to it.

The following chart compares beliefs about key doctrines as stated by open theists and Calvinists after "the period of controversy" between adherents of the two theisms began in 1994. During this period the "theology of open theism… rocked the evangelical world".

Contemporary open theists have named precursors among philosophers to document their assertion that "the open view of the future is not a recent concept," but has a long history.

The first known post-biblical Christian writings advocating concepts similar to open theism with regard to the issue of foreknowledge are found in the writings of Calcidius, a 4th-century interpreter of Plato. It was affirmed in the 16th century by Socinus, and in the early 18th century by Samuel Fancourt and by Andrew Ramsay (an important figure in Methodism). In the 19th century several theologians wrote in defense of this idea, including Isaak August Dorner, Gustav Fechner, Otto Pfleiderer, Jules Lequier, Adam Clarke, Billy Hibbard, Joel Hayes, T.W. Brents, and Lorenzo D. McCabe. Contributions to this defense increased as the century drew to a close.

The dynamic omniscience view has been affirmed by a number of non Christians as well: Cicero (1st century BC) Alexander of Aphrodisias (2nd century) and Porphyry (3rd century). God's statement to Abraham “Now I know that you fear me” (Gen 22:12) was much discussed by Medieval Jewish theologians. Two significant Jewish thinkers who affirmed dynamic omniscience as the proper interpretation of the passage were Ibn Ezra (12th century) and Gersonides (14th century).^{[citation needed]}