John Frame writes:
Now many have sought to discover the “essence” of Christianity- that which makes Christianity what it is, that without which it would be something else. Books have been written (as by Feuerbach and Harnack) entitled The Essence of Christianity. And many theologians, though writing books with other names, have sought, in effect if not in so many words, to identify the essence. The very variety of suggestions, however, casts initial doubt upon the project. What is the essence? Morality (Kant)? Religious feeling (Schleiermacher)? Philosophical dialectic (Hegel)? Wish-fulfillment (Feuerbach)? The fatherhood of God (Harnack)? Word of God (Barth)? Personal encounter (Brunner, Buber)? Acts of God (Wright)? The self-negation of being (Tillich)? Existential self-understanding (Bultmann)? Hope (Moltmann)? Liberation (Gutierrez)? Incarnation (Eastern orthodoxy)? Covenant (many Calvinists)? Five “fundamentals” (many American conservatives)? And what of holiness, justice, mercy, faith, love, grace, praise, spirit, peace, joy, body life? What of evangelism, worship? All of these have some claim to be called the “heart of the gospel” or the “center of Christianity.” Or why not say simply that “Christianity is Christ?”
His alternative to this type of theology is predictable for those who are familiar with his corpus, and I have no substantial improvement to offer over it. See here for his further comments.