Eckhart (c. 1260-1328), Dominican preacher and Paris master, was tried for heresy; Pope John XXII's bull In Agro Dominico (March 27, 1329) condemned 28 propositions drawn from his works, mostly from his German sermons. The condemned claims include the identity formulae: that the soul's spark is uncreated and one with God, and that we are transformed entirely into God as the bread in the sacrament. His German sermons, especially the detached sermons on Gelassenheit (releasement), are the primary texts; critical edition in the Kohlhammer Meister Eckhart series (Die deutschen und lateinischen Werke, 1936 onward).
Eckhart survives in popular culture as gentle aphorisms, 'if the only prayer you ever say is thank you', lines often apocryphal, which launder a radical into a greeting card. The historical Eckhart preached that one must abandon God for God, gelassenheit as the release of every image, including the image of God.
Eckhart's logic is a subtractive ascent: since God is beyond every predicate, the way is the progressive release of will, image, and selfhood, until only the ground remains, which was never created and never needed to be saved. It is the via negativa with the safety removed, which is what the bull objected to.
If the soul's ground is God's ground, who exactly is praying to whom?