If the Bible was not written or dictated by God but instead inspired by Him and written by men, how do we know that it is true? Why is the Old Testament the same for Christianity, Judaism, and Islam but the New Testament different for all three religions? How can Jesus be our Lord and Savior in one version but not in the other two? How do we know which two versions are wrong or if all three are wrong? Obviously, man does not have to write the truth if he is inspired by God so how can we trust that any of the Bible is correct and/or not corrupted by man?

You say that man does not have to write the truth if he is inspired by God. I assume you mean that if a man claims to be writing under divine revelation he can say whatever he wants, whether it’s true or not, as long as people believe he is really hearing from God. Actually, this is the case with both the Muslim Quran and the Book of Mormon. Each was written by one man who claimed to have a special revelation from God. The problem is there is no way to verify their stories except to take them at their word. In such cases we are left with having to weigh the internal coherence and external evidence. Is the message consistent and non-contradictory throughout? Are the facts and details it presents accurate based on what we know of science, history, geography and the like? Only the Bible–Old & New Testaments–can answer yes to these questions.

First of all, there is only one Old Testament (OT) that both Judaism and Christianity have in common. There is only one New Testament (NT) that belongs to the Christian faith. Islam didn’t exist until about 600 years after Christ. They believe Jesus was a prophet but they deny the teaching of the NT and rewrite OT history. In other words, they do not accept the OT or NT without changing them to agree with their holy book.

The OT primarily tells the history of God’s chosen people, from Abraham, to Moses, to the rise and fall of the Kingdoms of Judah and Israel. It particularly records God’s promises to Adam, Abraham, and David which would later be fulfilled through the coming of the Messiah.

The NT records the incarnation, life, death and resurrection of Jesus the Messiah and the ministry of his followers after he ascended to heaven.

The OT and NT were written by various people at various time in various styles using various genres of literature, yet they are remarkably coherent. They tell a single story of redemption through Messiah. They are written in history about historical events. So we can corroborate details from other non-religious historical sources. Especially in the case of the life of Jesus in the NT the events were recorded by eyewitnesses or by close associates of eyewitnesses. The NT is not a case of private revelation but of public record.

There is a lot to be said about how and why we can trust the Bible. Let me offer one of many reasons. We do not need to investigate everything the Bible says in order to trust it. If we investigate a few important details up front and find that what the Bible says on those topics is true then it will earn an increasing amount of trust. Trust does not require certainty. But a little trust goes a long way. It allows us to ask more questions, explore more doubts and allow the process to either increase or decrease our confidence in what the Bible says.

Again there’s SO MUCH to say about how very reliable the Bible is. I would recommend this video by Lee Strobel and Is the Bible intolerant? by Amy Orr-Ewing. It tackles some of the questions you have.