Serious contemplative teaching—very upfront in the desert fathers and mothers—is surely found in Celtic Christianity (outside of empire), and is continued by leaders of many monasteries, for example, by John Cassian (360–435 CE), Pseudo-Dionysius (5th–6th centuries), and Hugh of St. Victor (1096–1141) in Paris. Later mystics like Bonaventure (1221–1274), Francisco de Osuna (1497–1541), the unknown author of The Cloud of Unknowing (late 14th century), and 16th century mystics Teresa of Ávila (1515–1582) and John of the Cross (1542–1591) also taught nondual consciousness.
It held on much longer in the religious orders than the ordinary local church or with the common priest or bishop—whose ministry was an occupation more than a search for God or a “school for the Lord’s service,” as St. Benedict (480–547) described. 
.. after the over-rationalization of the 17th and 18th century Enlightenment, many of us Western Christians became very defensive, wanting to prove we were smart and could win arguments with the new secularism. We imitated the rationalists while using pious Christian vocabulary. It took the form of heady Scholasticism and rote formulas in Catholicism, and led to fundamentalism and memorized Scripture verses providing their own kind of “rationalism” among many Protestants.
.. Catholic doctrines (such as transubstantiation, papal infallibility, and hierarchical authority) came to be presented in a largely academic and juridical way (or, for the sacraments, with an almost magical interpretation), as opposed to a contemplative or mystical way.
.. Thomas Merton (1915–1968) was very influential in reintroducing contemplation to the West. Now it is again taught in Christian arenas all over the world under different names.
.. contemplation is the way you know and think of yourself when you are sincerely praying and present—as opposed to thinking, arguing, or proving.