Whoever feeds on my flesh and drinks my blood abides in me, and I in him. John 6:56
Meditation on Christ’s suffering was diligently practiced in the papacy, and it is still in vogue today. This is what they consider having Christ in their hearts. But by this they actually perverted the suffering of Christ and cast it to the ground. The Lord does not say: “Your thoughts of Me are in Me” or “My thoughts are in you” but rather: “You, you are in Me, and I, I am in you.” He does not refer to a mere thought, but He demands that I be in Him with body, life, soul, piety, and righteousness, with sins, folly, and wisdom; and He says that He, Christ, on the other hand, is also in me with His holiness, righteousness, wisdom, and salvation. Contemplation and shadowy and erratic thoughts, which are but an imaginary indwelling and nothing but thoughts, will not do here. It will not suffice if you can do no more than just discourse on Christ or reflect on Christ, for the devil and pope can do as well. They also converse about Christ’s sufferings, but that is nothing; for in spite of it they remain steeped in wickedness, in sins, and in their errors. They give no proof that Christ is dwelling in them; they do nothing that is good.
Luther’s Works, vol. 23, 145
Christ is serious when He says that you will abide in Him and He in you if you believe in Him. He says: “It matters not if you are still somewhat weak, for I am in you. If you lack anything, I have an abundance of righteousness, holiness, and wisdom; I have no weaknesses. But if you are weak, your weakness is in Me, and I will see to it that I help you, that I drown your weakness in My strength and power, that I delete your sin in My righteousness, that I devour your death in My Life.” This is the true meaning and significance, the sum and substance, of this text, that He abides in him who believes in Him.
Luther’s Works, vol. 23, 147-8