Is my repentance accepted after I jokingly said "I am an atheist" to someone who asked me to swear, and then I recited the two Shahadas and prayed the prayer of repentance?

Describing oneself jokingly as a Jew, Christian, apostate, or non-Muslim is considered disbelief (kufr), whether done in jest or out of ignorance. This is based on Allah's saying: "Say, 'Is it Allah and His verses and His Messenger that you were mocking?' Make no excuse; you have disbelieved after your belief." If the questioner repents and regrets what he said, we hope that Allah will forgive him, as Allah says: "Say to those who have disbelieved, if they cease, what has passed will be forgiven them."