We must first acknowledge that the difference between us and God is greater than the difference between us and, say, a bear. Then, imagine a bear in a trap and a hunter who, out of sympathy, wants to liberate him. He tries to win the bears confidence, but he can't do it, so he has to shoot the bear full of drugs. The bear, however, thinks this is an attack and that the hunter is trying to kill him. He doesn't realize that this is an attack and that this is being done out of compassion. Then, in order to get the bear out of the trap to release the tension on the spring. If the bear were semiconscious at that point, he would be even more convinced that the hunter was his enemy who was out to cause him suffering and chaos. But the bear would be wrong. He reaches this incorrect conclusion because he's not a human being. Now, how can anyone be certain that's not an analogy between us and God? I believe God does the same to us sometimes, and we can't comprehend why he does the same to us sometimes, and we can't comprehend why he does it any more than the bear can understand the motivations of the hunter. As the bear could have trusted the hunter, so we can trust God.
Some people think that faith means lack of doubt, but that's not true. One of my favorite Bible texts is about the man who comes to Jesus with his demon-possessed son, hoping that the boy would get healed. Jesus says all things are possible to those who believe. And the man's response is so powerful. He says, "I believe, but would you help me with my unbelief?" Oh, man! I can really connect with that! So doubt and faith can co-exist. It means you can have doubts even when you believe. That was even true of Abraham. He clearly believed, but at the same time, he had doubts. You can see that by what he did at times and what he said. Now, I don't know where you cross the line corrosive, eroding, there's probably no healthy faith.
I think doubt can actually play a positive role. I always get a little nervous at what I call the 'true believer' mentality, people with bright smiles and glassy eyes who never had a doubt in the world, who always think everything's wonderful, everything's great. I don't think they run in the same world I do. I'm afraid of what's going to happen to them when something bad occurs. For example, I know a physician whose four-year-old child was stricken with cancer. I remember many nights when forty of fifty people would jam into a house to fervently pray for that child. Some of them thought, "Of course he's going to be healed because we prayed." And he was not, it devastated them. Their theology had been misguided and unexamined. It had never been challenged by doubts or thoughtful questions. Doubts could have helped the developed a more substantial and realistic faith, to trust God in the face of death and not just in the face of healing.
You see, a faith that's challenged by adversity or tough questions or contemplations is often a stronger faith in the end.
On the whole, the gospels as excellent sources. As a matter of fact, they're the most trustworthy, complete, and reliable sources for Jesus. The incidental sources don't really add much detailed information; however, they are valuable as corroborative evidence.
Edit: It is quite difficult to know where to start, because actually the evidence for Jesus is so massive that, as a potential historian, I want to say we have got almost as much good evidence for Jesus as for anyone in the ancient world. Obviously there are some characters from the ancient world for whom we have statues and inscriptions. On the other and, we have statues of gods and goddesses in the ancient world too, and so you can never quite be sure. But in Jesus' case, the evidence all points firmly back to the existence of this great figure in the twenties through to around thirty of the first century. And the evidence fits well with what we know of the Judaism of the period, even though much of it was eventually written down a generation later, that I think there are hardly any historians today, who doubt the existence of Jesus. There are one or two. A man call G.A. Wells is the only one who made much of it recently. From time to time you get someone like J.M. Allegro, who a generation ago wrote a book on the basis of the Dead Sea Scrolls saying that Christianity was all about a cult of sacred mushroom. No Jewish, Christian, ath9est, or agnostic have ever taken seriously since. It is quite clear that in fact Jesus is very, very well documented character of real history. So I think that question can put to rest.