My question is that if someone having doubt in Islamic beliefs like Day of Judgment, Prophet, Heaven, Hell. Will he turn into a kafir? If yes, then what should he do now?
بسم الله الرحمن الرحيم
These are the fundamental beliefs, it is necessary to have firm belief in them. If someone has doubt then he should read لاحول ولا قوۃ الا باللہ and renew his faith by reciting Kalimah and must say that he is believing in these things without any doubt.
Allah knows Best!
Darul Uloom Deoband