How can one deal with a father who does not pray or fast, and sometimes utters words of disbelief when angry, while wishing to guide him and maintain filial piety?

Children must show kindness to their parents. Among the greatest forms of filial piety is striving for their guidance, which is in the hands of Allah the Almighty. The child is only obligated to exert the means, and among the most important of these are:

Supplicating for the parent: Allah the Almighty said: "And your Lord said: 'Call upon Me; I will respond to you.'" Advising with kindness: Because enjoining good and forbidding evil with a parent differs from doing so with others. Directing those whom the parent listens to: Such as scholars and virtuous individuals.

If you exert all these means, then it is not your concern whether the parent is guided or not. Advice is beneficial if there is hope for its usefulness. However, if the advice increases the parent's stubbornness, it is preferable to abandon it, based on the words of Allah the Almighty: "So remind, if the reminder should benefit."