How can one deal with a person who is drifting towards Christian missionary ideas, after previously having been involved in a Sufi tariqa under the pretext of "discovering them," knowing that he does not possess religious knowledge and cannot be prevented from doing so?

Your relative must refrain from associating with the Christian missionary if he does not possess knowledge with which to repel doubts, because exposing oneself to doubts is ruinous. If you are unable to prevent him, then you must supplicate for his guidance and invite him to seek knowledge and to learn about the truths of Islam and the deviation found in the distorted Christianity. We also advise you to consult the books of specialists in comparative religions, among them Dr. Munqidh Al-Saqqar, who has beneficial works available online.