Swearing by a religion other than Islam

Q: What is the ruling of Islam on a person who says: If I do so and so, I will be a Kafir (disbeliever), then he does it many times? This person regularly observes the five daily prayers and completes the reading of the whole Qur'an. It is worth mentioning that he declares the two Shahadah (Two testimonies of Faith) very often, makes Ghusl, keeps on offering Sunan (supererogatory acts of worship following the example of the Prophet) and seeks forgiveness of Allah, he still lives in constant worry. Did his past good deeds become worthless? Please, advise. May Allah reward you with the best!

A: It is not permissible for a Muslim to swear by a religion other than Islam, because the Prophet (peace be upon him) was authentically reported to have forbidden that. It is related in the two Sahih (authentic) Books of Hadith (i.e. Al-Bukhari and Muslim), that the Prophet (peace be upon him) said, Whoever falsely swears by a religion other than Islam on purpose will become as he says; (Part No. 23; Page No. 197) if he is true (in the thing he swears on), he will hardly return to Islam unblemished. Whoever breaks his oath must offer Kaffarah (expiation), repent to Allah and determine not to swear by similar oaths again. More importantly, he does have to expiate for breaking such oaths, and it is enough for him to repent and do righteous deeds. Allah (Glorified be He) says, And verily, I am indeed forgiving to him who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing them, (till his death). Moreover, his deeds will not become worthless because he did not intend Kufr (disbelief); rather he wanted to solemnly take it upon himself to do or not do something.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.