Rationalism is a reliance on reason as the best guide for belief and action. This sometimes can ignore any moral values accumulated through historical experience, leaving it all up to human reason to resolve observations of conflict and chaos in the world.
Karl Marx used rationalism to create communism which had ended up killing tens of millions of people in the 20th and 21st century and remains a dire threat in modern world. The Nazis likewise were socialists who embraced rationalism over God. In fact, some of the worst events in history were because of those exercising rationalism. Nuclear weapons for example; the constant threat of nuclear annihilation was invented by rationalists who allegedly were seeking to create a better and more peaceful world. None could ever guarantee that such weapons would not fall into the hands of morally depraved persons in the future who control governments, who have no regard for human life, or even non-state actors.
Most of human progress comes from conservative values that are skeptical of traditions that celebrate rebellion against God.  As Dennis Prager states, "Reason is just a tool. It is no more intrinsically moral than a knife." As he points evil can be just as rational as good. 
- Horatio is not a blind rationalist ... when he sees the ghost, he does not deny its existence — on the contrary, he is overwhelmed with terror. His ability to accept the truth at once even when his predictions have been proved wrong indicates the fundamental trustworthiness of his character.
Test all things; hold fast what is good. 1Thess 5:21 (NKJV)
The opposite of Rationalism is Romanticism.