This is a question that has been bothering me quite a bit for a long time. It can be easily found that there are differences in the four gospels on certain events. For instance, Jesus says to bring a donkey and its colt for the ride to Jerusalem in Matthew, while it is mentioned in the other 3 gospels that He asked for a colt only. In Matthew, Jesus gives a sermon on a mountain and the teachings of that sermon can also be found in Luke but nothing about preaching on a mountain is mentioned. Is it possible that Jesus repeated the Sermon on the Mount at different places in an elaborated way? There are some more of these differences between the gospels. How do we deal with such differences? I also want to know how Luke elaborated the teachings of Jesus on the mount–saying more than what can be seen in Matthew. The last thing I want to know is that how can Mark’s and Luke’s gospels be based on the original life of Jesus even though they were never with him? The book of Matthew and John makes sense because they were with Jesus and they received the Holy Spirit after the departure of Jesus from this world. Even if Mark and Luke were writing the gospel with a lot of studying and inquiry from witnesses, how can we be sure that their books were written in the guidance of the Holy Spirit? I believe that since both Luke and Mark were disciples/campanions of Paul (see 2 Timothy 4:11), they had the Spirit with them. What are your thoughts?
I have a large amount of material on this topic at the web site. We have entire courses on the reliability of the Bible. Perhaps you should consider taking our class from the Apologetics Certificate Program at the web site. Or you can do a word search on contradictions or on reliability at the site.
Let me give an extremely brief introduction to the topic as an answer to your question. Many claim contradictions in the Bible–especially in the gospels. Most of these supposed contradictions are very obviously not contradictions. Simply asking how the accounts could be consistent answers 90+% of such questions. The four gospels are independent accounts. If all the details were identical, then it would be obvious that the writers “colluded,” and that they were not independent witnesses. When we have different witnesses we will get different details because different people notice different things and consider different things to be worth recording. For example there were two demoniacs in Matthew 8:28-34, but only one in Mark 5:1-20. Is this a contradiction? No! There was one guy who was totally out of control, who was famous throughout the region, from whom dozens of demons were exorcised. There was another person, with perhaps only one demon (not sure how many). Apparently, he was not so absolutely out of control. He had not broken his chains multiple times. The demon or demons driven from him did not cause an entire herd of pigs to rush into the Sea of Galilee. Mark does not mention this other demoniac. This is no contradiction.
I have been a juror on three trials. Every trial involved eye-witness testimony. In all three trials, the eye witnesses reported very significantly different details. Yet, when we got into the jury room, all were agreed that the differences were not contradictions, but evidence of non-colluded eye witness testimony. If all had said the same thing, we would have been skeptical.
Of course, every case is different, but nearly all apparent “contradictions” fall into this category. Take for example the two accounts of the entry into Jerusalem. One mentions the donkey. The other mentions the donkey and its colt. Is there some requirement somewhere that demands the gospel writers to include every detail? No. Do the accounts contradict? No. Now, if one gospel said that there was a donkey, but no foal and the other said there was a donkey and a foal, that would be a contradiction. If one person told you that they saw me at a party and another said that he saw my wife and me at a party, would that be a contradiction? No. Both are true, and both accounts of the entry into Jerusalem are true. To legitimately charge a person with contradiction, the burden of proof is on the one making the charge. Difference does not mean contradiction. Yesterday Sean wore a blue shirt. Yesterday Sean wore a bluish green shirt. Yesterday Sean wore a blue shirt with white pinstripes. These are not contradictions.
About the Sermon on the Mount. I believe that Jesus preached literally hundreds of times. More likely than not, some of these sermons were fairly similar to one another. The accounts of Matthew, Mark and Luke are not transcripts of a single sermon. They are faithful accounts of what Jesus spoke. Could the Sermon on the Mount be a compilation of things preached on more than one occasion? Perhaps. You have to bear in mind the literary style of the ancient Near East. The question is not whether these are exact word-for-word transcripts of a single sermon, but whether these accounts are inspired and faithful accounts of what Jesus preached in Galilee. As for a sermon on a mountain versus a sermon on a plain, it is quite reasonable to imagine Jesus coming down to the foot of a mountain, in a meadow at the foot of that mountain, which would be an ideal location to make a speech to a large number of followers. There is no contradiction here at all. In fact, the two accounts make more sense together than apart.
As for Mark and Luke not being eyewitnesses, first of all, Mark was very likely an eyewitness of many of the gospel events. It is the consensus of scholars that it is likely he was the one who ran off nude from the arrest scene of Jesus. We cannot be sure how many of the events he witnessed. What we know from the early church fathers is that Mark was the constant companion of Peter—sort of like his right hand man. He also was intimately associated with Paul (2 Tim 4:11). If he was a spokesperson for Peter, as the early church fathers tell us, then we can assume that Peter approved the letter he wrote, and would have corrected any factual errors. In any case, this gospel was written in the 50s or at the latest in the 60s AD. If there were inaccuracies, then the apostles, including Peter, could have corrected these. Of course, as believers we know by faith that Mark in inspired by God, but we do not need this to conclude that the gospel of Mark is a faithful reflection of the oral tradition of his day, as well as with the multiple eye witnesses (including Mark himself!) still alive when he wrote.
With Luke, one can argue (as does Luke in his own book, when he tells us that he put together a more orderly account than that of those who came before him) that his is the most carefully constructed gospel, with the most attention to historical and factual detail. As a self-conscious historian, Luke includes many more specific details than Matthew, Mark and John. Historians have called Luke a historian of great ability. He tells us that he interviewed many of the eye witnesses to put together his account. Besides, there is not bona fide contradiction between Luke and the other gospels. By faith, I believe that his book is without error. If we do not include faith we can conclude that his gospel is a more or less reliable account of what Jesus said and did.
How can we be sure that the Holy Spirit guided them? I cannot prove this like a mathematical proof, but the consistency of the gospels, despite the fact that they are independent witnesses is one reason to believe in this. We cannot be “sure” in the scientific sense, but we have much factual basis for supporting our faith that these words are inspired. Then there is the general evidence of the Bible as a whole which supports this faith. The evidence strongly supports the inspiration of the entire Bible, but we can never prove that the specific verse Luke 11:14 is inspired. What kind of proof could we provide for this individual verse? What we have is the overall evidence for the reliability and inspiration of the Bible as a whole. This evidence is extremely strong. It is not a large leap of faith to the conclusion that “all Scripture is inspired by God.” Does this require some faith? Yes, but this is no huge leap of faith. Here is the bottom line for me, upon careful and balanced analysis, there is no clear contradiction in the four gospels. The evidence is that they are four independent accounts of the life of Jesus. Claims of clear contradiction do not hold up to scrutiny.