I respectfully and vigorously disagree with the above post. The Attorney's oath (at least that to which I swore when I was admitted to the bar) states in part:
I will never reject for reasons personal to myself, the cause of the defenseless or the oppressed, or delay any man's cause for lucre or malice.
The Canons of Ethics are crystal clear that an attorney's opinion of the guilt or innocence of his client is not a determinant. It that were the case, no person accused of a heinous crime would ever be able to retain counsel, or expect a vigorous defense. It should be pointed out that John Adams represented the British soldiers accused of manslaughter in the Boston Massacre because of his personal belief that everyone is entitled to an attorney, regardless of his perceived guilt or innocence. Similarly, Clarence Darrow represented Leopold and Loeb in the face of overwhelming evidence.
The quote above is misguided. It does not refer to an attorney's personal conscience when representing a client; but rather indicates that he/she should represent his/her client "zealously within the bounds of the law," to quote the relevant Canon of Professional Ethics.
In twenty years as a practicing attorney, there were many occasions when I represented one whose innocence I doubted. Some of those instances involved a paid retainer, other times I was appointed to represent the defendant by the court. Never did I--or any attorney worth his salt of whom I am aware--refuse to represent a client simply because of personal doubts. Only in instances in which he is too involved with the matter to render independent judgment should he step aside. If matters of conscience prevent lawyers from taking cases, then perhaps we must revert back to the days of the Court of Star Chamber.