Is it possible for a Christian lawyer to defend a known guilty client?
What lawyers focus on is not whether a client is guilty or not, but whether the government can prove his guilt beyond a reasonable doubt. In other words, the lawyer makes a distinction between actual guilt and legally being found guilty. See: "Representing a Client Whom the Lawyer Thinks Is Guilty." From the lawyer's viewpoint, people lie, so he doesn't know if an admission of guilt or innocence is really true or not. Thus, the lawyer's aim is not in getting the truth known but in forcing the government to have a solid case. In other words, a defense lawyer can see his job as one designed to make sure his client gets a fair trial.
For Christians, truth matters a great deal. Lawyers can refuse cases that they do not think they can do an adequate job in representing the client, which would be the case if the lawyer is certain the client is guilty. There may be times a lawyer is ordered to defend a client anyway. In such cases, the lawyer works to make sure that a fair trial takes place.
In one case that I witnessed, the lawyer did a good job explaining that "innocent" did not mean his client was without fault, it only meant that the prosection did not make a solid enough case, beyond a reasonable doubt, that his client violated the laws his client was charged with violating.
An interesting example is the adulterous woman in John 8. Those who brought the woman to Jesus had violated the law because the law required both the man and woman caught in adultery to be stoned (Leviticus 20:10; Deuteronomy 22:22). They only brought the woman. They even admitted to violating the law because they said they caught her in the very act of adultery. Jesus put the accusers on trial by stating that the ones without sin cast the first stone. This was in accordance to the law, that required the witnesses to a crime punishable by death be the first to cast a stone (Deuteronomy 17:7). Their problem was the realization that they could not claim to be sinless in their accusation. When all left, Jesus ask if there were any accusers left. Even though Jesus knew the woman was guilty of sin, by the law's requirement that there must be two or more witnesses, he made no charge (Deuteronomy 17:6). Instead he told her to stop sinning. Here was a woman who was in truth guilty but not legally found guilty.
Many thanks for your sound answer.