I appreciate that there is some religious/cultural aspect involved here, but will try to answer it from pure logic.
So can I be angry at them to make them realize their mistake?
- A: Being angry seldom helps anyone,
- B: it is unclear whether it is a mistake,
- C: they are autonomous, souvereign entities the same way you are.
So. Let's take this apart:
A: Even if you do decide that they are doing a mistake, and that they are not entitled to act on their own but that you must force your view on them, then anger will most certainly not help you. I don't know your parents, but from your question I'd expect them to be either indifferent or sad about your anger. I actually have never known anyone who was influenced in a positive way by anger. There is a slight chance to change other people by leading with positive example and such, but I don't really see how you can do this in this case.
B: Not every human will accept your view that forgivenness, even too eager forgivenness, is an error. You will most certainly find humans who forgive everything in the very second it happens. Speaking from experience, forgiving (all religious aspects aside) is very healthy for the psyche of the person doing it. On the other hand, holding a grudge and being angry can lead to very unhealthy results. So before you try to change your parents, you should really think this true and think about whether it won't do more harm in the end. Yes on the surface it may appear to you that they run the risk of getting fooled by the same evil persons more than once. But it may well be that their mental health would be much worse off if you somehow manage to change them.
C: Even if you do convince yourself that being angry is a worthwhile route to changing them, and that being forgiving is a mistake, you still have to consider that if you are entitled to decide your actions to be this, then they are entitled to decide their own actions and behaviour as well. Or, other way round, if you decide that they are not entitled to be treated as self-complete beings, then you are, by extensions not entitled either. So you are not entitled to try to change them, anyways.
TL;DR: be gentle, love your parents, and let them do as they wish. They are not harming you; and if they harm themselves, that is their prerogative as adult human beings. Be strong and supportive with them, not angry, and help them when they need help, not when you decide that they need it.