What should a person do if he committed kufr?

Answer

The one who has committed kufr should first regret in his heart, and acknowledge that his statement/act was kufr. Then he should recite the Kalimah Shahaadat once audibly to himself. Thereafter he should perform two raka'ts Salaatut Taubah and sincerely repent and ask Allah Ta'ala to forgive him for all the kufr he has done, whether he knows of it or not, or whether he remembers it or not. In addition, if he is married, he has to renew his nikah which could be done privately at home with just two witnesses. This will suffice.