What is the ruling on dealing with a Christian doctor, after disavowing other religions upon uttering the Two Testimonies of Faith? And is there a specific formula for the declaration of faith for one who wishes to renew his Islam, or will any mentioned formula suffice?

You must praise Allah for your repentance, and you must turn away from satanic whisperings by remembering Allah Almighty, reciting the Quran, and occupying yourself with what benefits you in your religion and worldly life, while increasing supplication, remembrance, and seeking Ruqyah Shar'iyyah (Islamic exorcism/healing). What is uttered by one suffering from obsessive-compulsive disorder does not take one out of the fold of Islam if it is due to obsessive-compulsive disorder that clouds the mind. Otherwise, it is disbelief, but repentance is accepted. The Shahadah (testimony of faith) is sufficient, along with sincere repentance, regret, and a firm resolve not to return to insulting. There is no harm in financial dealings with a Christian.