Does the statement made in jest, "Let's convert to the religion of the Sabians or Christ," constitute disbelief (kufr) or apostasy (riddah), and what are the consequences thereof?

You have committed a grave sin by suggesting disbelief (kufr) and commanding it, even if in jest, and this is considered apostasy from Islam. Allah Almighty says: "And if you ask them, they will surely say, 'We were only indulging in idle talk and amusement.' Say, 'Is it in Allah and His verses and His Messenger that you were mocking?' Make no excuse; you have disbelieved after your belief." Sincere repentance is obligatory through regret and the resolve not to return to it, uttering the two testimonies of faith (Shahadatayn), and performing ghusl (definition) (ritual bath).