Within the context of church history, it means opinions that go contrary to God's revealed word.
Rationalism seeks a natural as against a supernatural explanation.
In this perspective, man's reason sits in judgment on God and his revelation instead of working in terms of it. Such rationalism may agree with orthodox theology, but in its basic premise, the sufficiency, right, and necessity of reason to sit in judgment over God and his revelation, it is heresy. It enthrones man's reason as the supreme judge over all things, beginning with God. Rationalism has insisted on identifying itself with reason, a fallacy, because it represents a misuse of reason. It can recognize and approve of the God of the Bible but only as an independent source of judgment and corroboration.