I posted the following in the conviction that he's not, but I think the fairest statement is that I don't know. I haven't read him thoroughly enough, and his style makes it impossible for me to want to read him more thoroughly.
I'm adding this caveat now because I don't know if the writer I've quoted below really knows for sure what Bonhoeffer means either, and the quotes may be somewhat out of context. All I can say is that I too had the same impression when I read Bonhoeffer myself.
It's always bothered me that Dietrich Bonhoeffer is so often admired and quoted by Christians, because he was not a true believer. Hearing him quoted by Christians does create the impression that he is one of us, and gives him a status he doesn't deserve which can mislead people. He courageously opposed the Nazi regime* and he said some memorable things that are useful for the church ("Come to Christ and die" or some version of that), but anyone who is led by those quotes to study him further will find that he was a "liberal" Christian at the very least.
This discussion reveals more about his apostate theology than I had known and I knew enough to be concerned already:
1. He believed that "God is teaching us that we must live as men who can get along very well without Him. The God who is with us is the God who forsakes us." Bonhoeffer also believed that the concept of God as a "supreme Being, absolute in power and goodness," was a "spurious conception of transcendence," and that "God as a working hypothesis in morals, politics, and science ... should be dropped, or as far as possible eliminated" (Letters and Papers from Prison, S.C.M. Press edition, Great Britain: Fontana Books, 1953, pp. 122, 164, 360).==================================
2. He believed that mankind had become of age and no longer needed religion, which was only a deceptive garment of true faith; he suggested the need for a "religionless Christianity." To Bonhoeffer, "the Christian is identified not by his beliefs, but by actions, by his participation in the suffering of God in the life of the world" (Letters and Papers from Prison, S.C.M. Press edition, Great Britain: Fontana Books, 1953, p. 163). Thus, Bonhoeffer's final writings have given impulse to Marxist theologians sponsoring "liberation theology" and to others wishing to promote a worldly social gospel.
3. He refused to discuss the origin of Christ, His relationship to the Father, His two natures, or even the relationship of the two natures. Bonhoeffer was adamant in his belief that it was impossible to know the objective truth about the real essence of Christ's being-nature (Christ the Center, pp. 30, 88, 100-101).
4. He questioned the Virgin Birth, and in reality denied it (The Cost of Discipleship, p. 215).
5. He denied the deity of Christ; he advocated that "Jesus Christ Today" is not a real person and being, but a "corporate presence" (Testimony to Freedom, pp. 75-76; Christ the Center, p. 58).
6. He denied the sinlessness of Christ's human nature and further questioned the sinlessness of His earthly behavior (Christ the Center, pp. 108-109).
7. He believed that Christ exists in three "revelatory forms" -- as Word, as sacrament, and as church. From asserting that Christ is the church, he followed that all persons in the church are identical with Christ (Christ the Center, p. 58; The Cost of Discipleship, p. 217). This amounts to pantheism!
8. He believed that Christianity is not exclusive, i.e., that Christ is not the only way to God (Testimony to Freedom, pp. 55-56).
9. He was a prominent figure in the early ecumenical movement, as evidenced through his associations with the "World Alliance for International Friendship" (a forerunner of the apostate World Council of Churches [WCC]), Union Theological Seminary, and Visser 't Hooft (who later became the first General Secretary of the WCC) (Testimony to Freedom, pp. 22, 212, 568). Bonhoeffer also reached out to Roman Catholics, prefiguring the broader ecumenism that blossomed after Vatican II in the mid-1960s.
10. He was a practical evolutionist (No Rusty Swords, p. 143), and believed that the book of Genesis was scientifically naive and full of myths (Creation and Fall: A Theological Interpretation of Genesis 1-3).
11. He adhered to neo-orthodox theology and terminology concerning salvation (Testimony to Freedom, p. 130), was a sacramentalist (Life Together, p. 122; The Way to Freedom, pp. 115, 153), believed in regenerational infant baptism (Letters and Papers from Prison, Macmillan, pp. 142-143) as well as adult baptismal regeneration (The Way to Freedom, p. 151), equated church membership with salvation (The Way to Freedom, p. 93), and denied a personal/individualistic salvation (Letters and Papers from Prison, Macmillan, p. 156).
12. He placed little or no value on the Old Testament --"... the faith of the Old Testament is not a religion of salvation" (Letters and Papers from Prison, S.C.M. Press edition, Great Britain: Fontana Books, 1953, p. 112).
13. He denied the verbal-plenary inspiration of Scripture, believing that the Bible was only a "witness" to the Word of God and becomes the Word of God only when it "speaks" to an individual; otherwise, it was simply the word of man/men (Testimony to Freedom, pp. 9, 104; Sanctorum Communio, p. 161). To Bonhoeffer, the Bible was meant "to be expounded as a witness, not as a book of wisdom, a teaching book, a book of eternal truth" (No Rusty Swords, p. 118). He also believed in the value of higher criticism/historical criticism, which is a denial of the inerrancy and authenticity of the Bible (Christ the Center, pp. 73-74).
14. He had no faith in the physical resurrection of Christ. Bonhoeffer believed the "historicity" of the Resurrection was in "the realm of ambiguity," and that it was one of the "mythological" elements of Christianity that "must be interpreted in such a way as not to make religion a pre-condition of faith." He also believed that "Belief in the Resurrection is not the solution of the problem of death," and that such things as miracles and the ascension of Christ were "mythological conceptions" as well (Christ the Center, p. 112; Letters and Papers from Prison, S.C.M. Press edition, Great Britain: Fontana Books, 1953, pp. 93-94, 110).
* although joining the plot to assassinate Hitler was not right from a Christian point of view.