A friend of mine said “I was actually about to give the gospel a try until you told me I’m burning in hell” sarcastically to a Christian who was trying to convert him. If you say statements of kufr sarcastically, do you become kafir?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It does not appear that this word contains mockery of religion; in any case, you must avoid whispers, just as you must avoid what could be mockery of religion; for mocking Allah, or His Messenger, or His Verses is very serious.

Allah Says (what means): {And if you ask them, they will surely say, "We were only conversing and playing." Say, "Is it Allah and His verses and His Messenger that you were mocking?" Make no excuse; you have disbelieved after your belief.} [Quran 9:65-66]

Allah knows best.