Assalaamu alaykum wa rahmatullaahi wa barakaatuhu. Please let me know; if a person does an act which he thinks may be mockery of the religion yet he continued doing it but did not intend mockery by it. Is there a possibility that it may be disbelief?

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. 

Mockery of the religion that is considered Kufr (disbelief) is the one that is intended to mock the religion. If one did not intend to mock the religion, then this is not considered Kufr.

Shaykh Ibn ‘Uthaymeen was asked, “A person imitated the prayer: for example, people are sitting in a meeting, and a person said the Takbeer [saying Allahu Akbar] – for example – as if he is praying the funeral prayer and recited Soorah Al-Faatihah.” The Shaykh interrupted, “Is it done for teaching or what?” The questioner said, “For nothing; is this considered mocking the prayer?”

He replied, “This is not mockery unless he meant to mock, because this may be intended for teaching. If it is not intended for teaching; is it mockery or not? We say: in principle, it is not mockery.” [Excerpted from the Open Door Meeting – Liqaa’ Al-Baab Al-Maftooh]

You should know that if a person’s Islam is confirmed with certainty, it is not removed from him except with similar certainty. As long as there is probability and doubt, then in principle his Islam is still valid.

Al-Mulla ‘Ali said in Sharh ash-Shifaa’, “Our scholars said, ‘If you find ninety-nine reasons to accuse a Muslim of Kufr and you find one single reason to keep him in Islam, then the Mufti and the judge should work based on that one reason, and this is implied from the hadeeth that reads, 'Avert the Hudood (the prescribed punishments determined in the Shariah) against Muslims as much as possible. If you find a way out for a Muslim, then let him go, for the Imaam making a mistake by forgiving is better than him making a mistake by punishing.' [At-Tirmithi and Al-Haakim]’” 

For more benefit, please refer to fatwa 328495.

Allah knows best.