The short answer to this, is that we believe the whole Bible to be true because Jesus did. If he is the Son of God as he claimed, then he alone is able to give us a correct view of the Bible. He regarded its writing to have been overseen by God to such an extent that it portrays just what he wants it to. We cannot therefore accept the entire Bible as true until we have first come to a decision about Jesus.
Nevertheless, a number of things still need to be said: First, the gospels can be established as historically accurate enough to come to conclusions about Jesus. Second, the process of translation is extremely accurate. Translators go back to the earliest copies to ensure that what we have is not distorted down the years. Third, if we accept for a moment that there is a God who created and sustains all things, it is certainly possible for him to ensure this book portrays just what he wants. Fourth, if we further assume that he is communicative, because we, his creatures, are communicative, then it is actually quite plausible that God would communicate with us. And there is no better means, that writings that can be passed down through time, checked, studied, and discussed. Fifth, an unprejudiced reading of the Bible itself testifies that it is more than merely a human book. It was written over 1500 years by 40 or so authors. Yet its message is coherent, consistent, and its teaching profoundly describes the world as it is, and transforms lives like no other. Sixth, there are actually far less apparent contradictions in the Bible than many claim there are. Moreover, a closer inspection of those that do seem to be there usually reveals that they are not contradictions at all, but different perspectives on the same events which can quite easily be harmonized. We should also be humble enough to recognize that the reason two passages might seem to contradict is because we have not fully understood their circumstances. Seventh, a key reason the Bible’s trustworthiness is doubted is simply because we find the things in it hard to believe or accept. Yet the reason we find them hard may well be because we have misunderstood them. And where we have not, it does not follow that because things seem unbelievable and unacceptable to us, in our particular culture, they are necessarily wrong.(John 5:37-40, 2 Timothy 3:14-17, 2 Peter 3:15-16)