This is not an easy book to read. It is meat, not milk. Yes, you do have to be a saint, i.e. a believer, i.e. a disciple in order to be a Christian. Having faith means being faithful. Having faith means bearing fruit. Having faith means being a disciple. Calling Jesus, "Lord" means obeying Him. Jesus, not Bonhoeffer, not the RC Church nor Luther nor Calvin nor whoever else may "speak to you" is the One Who says, "Take up your cross and follow me." Pastor Bonhoeffer leads the reader through some fairly profound meditations on discipleship. One does not simply read this book as though it were a magazine story; he must read and re-read and re-read again passage after passage as he works his way through it.
If you are quite comfortable in knowing you are "saved by grace through faith," and freed from the tyranny of the works of the flesh and the Law, wonderful. But ask yourself whether Jesus suffered and died on the cross just so you could be comfortable in your "faith." If it occurs to you that there may be more to faith than creeds, confessions, and church attendance, that you were given the gift - and gifts - of the Spirit for some purpose other than as a voucher for your personal ticket to Heaven, you may find this book very helpful. In any event, should you read it, it will challenge you in many marvellous ways.
It is quite difficult, I think, to fully appreciate what the man wrote without understanding and contemplating what was going on (and eventually transpired) in his life, church, country, and world. Thus, I recommend that anyone who reads this learn as much about these other things as he reasonably can, for which purpose there are many options, including books, articles, and films on video and DVD.
The last fourth of the book was a little "thicker" to me, and I did not get as much out of it. The author's thoughts were not as lucid once he got out of the Sermon on the Mount passage, and they did not communicate as much to me personally. However, the first 3/4 of this book is so powerful that I would recommend it to anyone.
Be prepared to face your comfortable habits and ways of thought in a new light if you read this book. The Word is a sword, and Bonhoeffer uses it to penetrate us.