How can faith and piety be firmly established in the heart after repeatedly apostatizing from Islam and returning to it, which is believed to be due to previous beliefs?

The reality of Islam is submission to Allah alone and obedience to His Messenger (peace be upon him). The foundation of its religion is the testimony that there is no god but Allah and that Muhammad is the Messenger of Allah. Whoever adheres to this until death will be among the people of Paradise. And whoever does not embrace it, or apostatizes from it, and dies upon disbelief will be among the people of Hellfire. There is concern for one whose apostasy recurs that he may not be guided to repentance, due to the Almighty’s saying: (Indeed, those who believed, then disbelieved, then believed, then disbelieved, then increased in disbelief - never will Allah forgive them, nor guide them to a way). One must hasten to repent and remain steadfast in Islam, observe the obligatory duties, recite the Qur'an, read books that foster love for Islam, distance oneself from bad companions and arguments in matters of religion, and strive against one's self in obedience to Allah.