Throughout the novel, Robinson Crusoe struggles with what he has believed in his life and what he now believes to have discovered as truth. The most important of these is his relationship with religion and God; before his misfortunes, he was ambivalent at best towards religion, and saw little purpose to prayer or faith. In his mind, his own desires and needs were paramount, and there was no higher power to change his fate.
During a long and almost fatal illness, Robinson has visions of an angel of death, who berates him for not recognizing his God-given fortune in surviving the wreck on a livable island, and with access to supplies. He makes the conscious decision to change his ways, and thereafter becomes devout, both in his private life and in proselytizing.
I had hitherto acted upon no religious foundation at all; indeed, I had very few notions of religion in my head, nor had entertained any sense of anything that had befallen me otherwise than as chance, or, as we lightly say, what pleases God, without so much as inquiring into the end of Providence in these things, or His order in governing events for the world.
(Defoe, Robinson Crusoe, eNotes eText)
In this manner, "truth" as a personal acceptance of God's will is seen in Robinson's actions; he stops trying to change or thwart God in his life and, after accepting that there might be a fate for him beyond that which he personally and selfishly desires, begins to achieve more in his survival.