What is the justification for relying on faith healing over conventional medicine? Every so often, I read about Christian parents choosing faith healing over conventional medicine for their sick children, in some cases with lethal results. This usually results in the parents being convicted of either severe neglect or manslaughter. (Rightfully so, in my opinion)

I know there are several accounts of miraculous healing mentioned in the Bible, and I don't see a problem with prayer in addition to medicine, but what is the justification for opting to rely on prayer alone? 

Is the answer different when it's an adult making the choice for themselves rather than for their child?
DISCLAIMER: I do not agree with this justification, I am merely answering your question.

The justification for relying on faith healing over conventional medicine comes from a belief that turning to man is diametrically opposed to turning to God.  In other words, you can either turn to God for help, or you can turn to man for help - but not both.  One passage used to support this is found in Psalm 118:


  It is better to take refuge in the Lord than to trust in man. -Psalm 118:8


One of the main passages used to support the application of this doctrine to healing is a story in the Old Testament about a king named Asa:


  In the thirty-ninth year of his reign Asa became diseased in his feet. His disease was severe, yet even in his disease he did not seek the Lord, but the physicians. So Asa slept with his fathers, having died in the forty-first year of his reign. -2 Chronicles 16:12-13


The "faith-healing" advocate would say that Asa's downfall was in turning to the doctor, and that we should turn to the Lord instead.

Of course, if you believe that turning to God can include receiving help from man, you would interpret the passage differently.  But now it should make more sense where this doctrine comes from.