The sixth (and last) question is "the Gospel and doctrine, or the question of creed." I touched on Harnack's disjunction between doctrine and life/experience in one of my early posts. He expands on that theme here.
Harnack starts right off by rejecting doctrine in favor of ethics:
The Gospel is no theoretical system of doctrine or philosophy of the universe; it is doctrine only in so far as it proclaims the reality of God the Father. It is a glad message assuring us of life eternal ... By treating of life eternal it teaches us how to lead our lives aright. (p. 146)
Now it might seem that doctrinal confession of some sort is very important. After all, Jesus said, "Everyone who confesses me before men, I will also confess him before my Father who is in heaven" (Matt 10:32 NASB). But, according to Harnack, Jesus here means ...
the confession which shows itself in feeling and action ... It is religion and the moral element that are concerned ... 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 ... Creed is to be nothing but faith reduced to practice. (p. 147-48)
Harnack's reduction of creed to practice/ethics/life is crucial to his whole understanding of Christianity. It is what enables him to sweep aside the doctrinal disputes of church history and to seek to return to the "simple" religion of Jesus, which focused on the Fatherhood of God and the infinite value of the human soul, as well as the law of love.
So I have finished going through Harnack's six objections to Christianity. This brings us to the half-way point of his lectures. The remaining lectures survey the development of the Christian religion in history in five stages:
(1) In the Apostolic Age
(2) In its Development into Catholicism
(3) In Greek Catholicism
(4) In Roman Catholicism
(5) In Protestantism