All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
We do not know the facts about those Christian therapists to whom you referred in the question. However, generally speaking, when the sick person is granted recovery from his illness at the hands of a therapist through Ruqyah (Quranic healing) or another means of treatment, this does not indicate the soundness of the therapist's beliefs (or his piety). A Christian therapist may use Quranic verses and reported supplications in treating a patient with the Ruqyah and Allaah may grant the patient recovery from his illness. Moreover, a Christian therapist may treat a physical illness by means of a surgery and Allaah may bless the patient with recovery as well.
Even if the sick person is granted recovery from his illness due to extraordinary means (used by the therapist), this does not indicate the soundness of the therapist's beliefs at all. It is possible that Allaah causes extraordinary matters to take place through unusual causes at the hands of His enemies, such as magicians and others. However, this is intended to be a means to lead them gradually to their own destruction and for them to indulge further in error. It may be a trail for others as well. For more benefit, please refer to Fataawa 18341 and 86103.
You should know that preoccupying oneself with knowing the details of how devilish supernatural events take place at the hands of some therapists is of no benefit. Believing that such people have gone astray does not depend on this information.