We first see Ishmael's Christianity and Christian morality portrayed in the very first chapter, especially in the famous opening line, "Call me Ishmael." In Hebrew, the name Ishmael can be translated as "God has hearkened," meaning that the name implies divine promise or prophecy to be fulfilled. Melville's character Ishmael is symbolically named for the character found in the Bible. In Genesis Chapter 16, Ishmael was born of Abraham and Hagar, the maid of his wife Sarah. Sarah had granted Abraham permission to have a son with her maid in order to fulfill God's prophecy for baring many descendents despite the fact that Sarah was infertile. God blessed Ishmael, promising to make him a great nation. Despite God's promises for Ishmael, Ishmael was cast out of Abraham's household and sent into the dessert to parish, but God saved him and still upheld his promise to make Ishmael into a great nation.
Similarly to the biblical Ishmael, it's clear that Melville's Ishmael also undergoes many dispiriting tribulations. In fact, we learn in this first chapter that Ishmael makes the decision to sail at sea whenever he starts feeling so depressed he thinks of committing suicide. Since we know he would prefer to distract himself with a change of scenery rather than commit suicide, we know that he also highly values the morals he has learned through is Christian upbringing. The Christian religion condemns the act of suicide as an act of trying to take into one's own hands God's power of both giving life and taking life. Hence, even from this very first chapter, we learn that Ishmael strongly values the Christian religion.
We first see Ishmael's contradictory beliefs concerning religion start to develop in the first few chapters of the book as well. In Chapter 3, Ishmael is obligated to share a bed at the inn with Queequeg, a man who is a cannibal from the uncharted island of Kokovoko. Even though Ishmael was at first terrified of Queequeg due to his fully tattooed exterior and his threat to kill Ishmael if he does not explain what he is doing in Queeqeg's room, Ishmael and Queequeg actually become good friends. More importantly, Ishmael observes that once the situation was explained to Queequeg, Queequeg, the pagan, was not only civilized but even kind and charitable, even more kind and charitable than any Christian might be in a similar situation. We see Ishmael's descriptions of Queequeg's kindness in the passage:
"You gettee in," he added, motioning to me with his tomahawk ... He really did this in not only a civil but a really kind and charitable way ... Better sleep with a sober cannibal than a drunken Christian. (Ch. 3)
Hence we see that the moment Ishmael befriends a pagan cannibal is the moment that his views on Christian ethics and behavior start turning upside down. If Ishmael can observe that a pagan cannibal can behave more decently than many Christians, then surely he would start having contradictory thoughts concerning the superiority and righteousness of his own religion.