Martin Rumscheidt has a helpful overview of Harnack's life that you can read on Google books (Adolf von Harnack: Liberal Theology at Its Height [San Francisco: Collins, 1989], 9-33). In his introduction, Rumscheidt quotes Harnack as saying:
I do not desire to be given the fulness of ready-made statements, rather, I want to produce every statement in that web by myself and then make it my own.
Rumscheidt then comments:
In this are the roots of Harnack's theological liberalism: the need to understand, to know religion historically and to relate it to all other historical phenomena, to push beyond the merely dogmatic or creedal to the living core or essence of religious truths and then to defend the self-won insights with frankness, integrity and reason (p. 11).
Harnack's dissatisfaction with "the merely dogmatic or creedal" approach to the Christian religion, and his drive to push on "to the living core or essence" of the faith is seen clearly in What is Christianity? For example, Harnack says:
An experience -- it is only the religion which a man has himself experienced that is to be confessed; every other creed or confession is in Jesus' view hypocritical and fatal. (p. 148)
It is not a question of a "doctrine" being handed down by uniform repetition or arbitrarily distorted; it is a question of a life, again and again kindled afresh, and now burning with a flame of its own ... Individual religious life was what [Jesus] wanted to kindle and what he did kindle. (p. 11)
The distinction between "doctrine" and "life" (as a religious experience) is not necessarily bad, but Harnack takes it to great extremes.
For example, he has no time for the "theories about vicarious sacrifice," but concludes that Christ's death was "for us" in some vague sense that we can "feel" but not articulate (pp. 156-60).
Or take another example--the "Easter message" (read "doctrine") of the empty tomb is not important; what is important is the "Easter faith" (read "experience") of the living Christ (p. 160).
Thus for Harnack Christianity is essentially a religious experience. The doctrinal truths that the church has historically proclaimed and confessed are irrelevant and in many ways dangerous, since they obscure the immediacy of the religious experience or feeling that is, in some ways, a more authentic way of grasping the truth than the noetic development of doctrine.
Harnack, then, becomes an iconoclast who is continually attacking the church's traditional creedal formulas as he tries to uncover the essential core experience that lies beneath the mass that has grown up like barnacles upon the hull of a ship. Or to use another analogy, he attempts to peel off the layers of the onion, only to be left with nothing in the end.