If Deists believe in God, why do they reject Christianity, Judaism, and Islam?
Deists believe that revelations from God, if there are true revelations from God, can not be passed on from the person getting the revelation to another. Due to the limits of language, one person can never convey something as personal as a revelation from God to another person. Most Deists also recognize the inconsistencies in the holy books of the major religions. Deists believe the best way to determine the nature of God, is through nature. Nature is universal and does not have any need for translation.
Deists doubt that there are truly revelations from God. If God did choose to reveal anything to a human, that person could not reliably pass that revelation on to another person. Due to the limits of language, one person could never convey something as personal as a revelation from God to another person. Most Deists also recognize the inconsistencies in the holy books of the major religions. Deists believe the best way to determine the nature of God is through His creation. The language of nature is universal, and so it has no need of translation.