Does doubting the authenticity of a miracle of the Prophet, peace be upon him, for reassurance, expel one from Islam? And will Allah forgive these sins?

A Muslim must praise Allah for the blessing of Islam. Doubts may afflict him due to his limited knowledge or weak faith, so he must ask people of knowledge and ponder the Quran and the Sirah (Prophet's biography) to increase his faith and tranquility. There is no harm in asking about miracles to increase certainty, but he should beware of persisting in doubt and abandoning good deeds. Allah forgives sins through repentance, no matter how great they are, so he should turn to the Quran and supplication. If the doubt is based on a specious argument, he should ask people of knowledge. If it is a whisper (waswasah), its cure is through remembrance of Allah (dhikr), obedience, seeking refuge in Allah, and ignoring it.