Added 1.17.16
You say you believe in Abraham, Moses, and Jesus, so why don't you take the Bible as guidance?
Muslims believe that everything Abraham said and everything Moses said and everything Jesus said, peace be upon all three of them, is true. When it comes to the Bible, though, everyone should take some factors into consideration as to the level of accuracy and creditably with which these messages have reached us. The modern New Testament (NT) versions are composite translations from over 3200 Greek manuscripts/fragments, 8000 in Latin, and about 1000 in various other languages. Those copies differ in various ways, so translators made choices in favor of certain ones over others. As to the accuracy level of the copies themselves, since they are not the originals (lost), that's an unknown. On top of that, there is only one verse in the entire NT available in the language Jesus spoke (Aramaic). This is not the picture of reliability already. Moreover, the books themselves do not appear to agree in their messages. Compare, for instance, James, with the ideology Paul, a former persecutor who says he met Jesus in a vision. The division between the thinking of Paul and Barnabas is also well documented. What's not documented well are the histories or even identities of many of the other writers, including even Matthew, Mark, Luke, and John. The canonization process, as well, took place long after the ascension of Jesus, and could be criticized from multiple standpoints. As for the trinity doctrine, specifically, The New Catholic Encyclopedia states, "It was the product of 3 centuries of doctrinal development." At a point as a non-Muslim I doubted the message of Jesus accurately wound it's way through that maze. Now, as a Muslim, I know it did not. Allah says, "And [beware the Day] when Allah will say, 'O Jesus, Son of Mary, did you say to the people, 'Take me and my mother [note Catholic veneration, etc.] as deities besides Allah ?'' He will say, 'Exalted are You! It was not for me to say that to which I have no right. If I had said it, You would have known it. You know what is within myself, and I do not know what is within Yourself. Indeed, it is You who is Knower of the unseen'" [al-Qur'an 5:116]. Furthermore, there are several ayaat and ahadith which make it clear al-Qur'an and as-Sunnah are sufficient guidance without resorting to other religious books to define Islam, though apparent wisdom may be taken from any number of places as we go through life.
As for the Old Testament, I find great relief in knowing the major sins of prophets are lies, such as what is attributed to David, Solomon, and Lot, peace be upon all of them. I'll let Christians, themselves, interpret Jeremiah 8:8: "How can you say, 'We are wise, for we have the law of the Lord,' when actually the lying pen of the scribes has handled it falsely?" However, I affirm what Allah says of the early Jews in al-Qur'an 5:13, which reads, "They change the words from their places and have abandoned a good part of the Message that was sent to them. And you will not cease to discover deceit in them, except a few of them. But forgive them, and overlook (their misdeeds). Verily, Allah loves Al-Muhsinun (good-doers)."