When discussing the Gospels with skeptics, many times skeptics will bounce back and forth between different positions of skepticism (one minute they accuse the gospel writers of collusion, the next minute they say the Gospels are full of contradictions, and the next minute they say the text is corrupted, i.e., we have no idea what the original writers wrote). In short, the skeptic can't get the reason for his skepticism straight; as a result, it's difficult for the conversation to gain traction. To prevent them from these schizophrenic chameleon tactics, the following trilemma will force them to take a position rather than just play the role of mocker and heckler.
Either the gospel writers were delusional men who coincidentally all experienced the same delusion, were unscrupulous opportunists who were trying to spread lies (for reasons beyond historical and rational comprehension), or men of integrity committed to the truth. In other words, the driving force behind the gospel accounts was one of the following:
1. Confusion and with it contradiction--
This view takes into account the swoon (Jesus didn't really die) theory, imposter (the disciples were fooled by an imposter) theory, and hallucination (the disciples imagined the resurrection) theory. In all incidences it's safe to say the disciples were pretty confused. If this is the theory the skeptic wants to cast his lot, then he can't trust the reliability of the witness; therefore, he can't just pick and choose which is a reliable/authentic saying of Jesus Christ and which is not. For example, the skeptic (Muslim, atheist, etc.) can't pick the verses of Jesus saying, "the Father is greater than I" in John 14:28 as evidence against His deity, while at the same time denying what Jesus said about the Holy Spirit in John 14:26--"But the Advocate, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of everything I have said to you." If they trust the accounts of what Jesus said regarding the Father they should trust the accounts of what He said about the Holy Spirit. Accepting one statement as authentic and authoritative while dismissing another as reliable, when both statements come from the same source of authority, is nothing more than cherry-picking statements that affirm what you are trying to prove. This is not truth-seeking, this is circular reasoning.
"Do we have any good reason to believe that the apostles were driven to lie by [greed, sexual desire, or pursuit of power]? No. There is nothing in history (neither Christian history nor secular history) to suggest that the disciples had anything to gain from their testimony related to Jesus...As we examine the motives of the gospel writers, it's clear that the forces that typically compel people to lie didn't drive the authors."
--J. Warner Wallace (former atheist, and homicide detective who researched the "chain of custody" i.e., transmission of the Gospels, used his area of expertise--forensic statement analysis--and applied it to the gospel accounts, and determined their reliability. He documented this analysis in the book, Cold-Case Christianity).
Additionally, if collusion is the theory the skeptic wants to rely on, there can be no appeal to contradictions (since part of the goal of colluding to tell a story is to tell a consistent one), and just like the confusion theory the skeptic can't trust ANYTHING, so he can't just pick and choose which sayings are authentic and which are part of the collusion conspiracy theory.
3. Commitment to reporting the truth (of what they saw and heard)--
If this is the case, the gospel writers were committed to reporting the acts, sayings, and life of Jesus as accurate as possible; therefore, among four different authors one would expect variation and even paradoxes, but not contradictions. Furthermore, the writers considered who they were writing to in order to emphasize the details of Christ that would be most relevant to them. Matthew’s Gospel was written to the Jewish people of his day to tell them the long-awaited Messiah has come; Mark’s Gospel was written to the people in Rome; Luke’s Gospel was written to a person named Theophilus; John’s Gospel was written to the Greco-Roman world/gentile Christians and emphasized Jesus Christ is the fully divine Son of God who existed before Creation, and is the true Lamb of sacrifice through whom we receive the gift of eternal life. The unequivocal language John used throughout his Gospel was so that it could not be misunderstood that Jesus Christ was the one true God of the universe.
3a. Corruption of the texts (i.e. the integrity of the gospel writers is not doubted; however, the transmission and the copies have been corrupted and therefore we don't know for sure what the original writers wrote.) The only ones making this argument are ill-informed skeptics who do not know the New Testament documents are the most well attested documents of antiquity, or those presupposing the view they already hold into the text (e.g. Muslims). We answer 3a. in depth here. The three minute video below of Shabir Ally, a Muslim scholar, demonstrates how Muslims (like Shabir) use circular reasoning when it comes to interpreting the Gospels.
Since we can't see, hear, taste, touch, or smell the past, the highest degree of certainty to which we can arrive regarding what happened in the past is that of probability (i.e. what explanation is more likely given all the facts?). When considering all the indisputable facts of Jesus and the origin of the disciples' belief in the resurrection, one can see how ridiculous every confusion, delusion, and conspiracy theory is. There is simply no CAUSAL connection (historical evidence) to back up any of these theories; thus, you not only have to deny the evidence we do have, but you have to rely on speculation just so the conclusion can have some semblance of logical consistency. The only reason to deny view 3 as the most probable explanation of the facts is if one assumes no personal God exists and therefore miracles are not possible (i.e. uses circular reasoning), or if they presuppose their worldview (as we showed how Muslims do) as more accurate than historical unanimity. Some reputable scholars who are not Christian may hold views 1), 2), or 3a) implicitly in their conclusion (since they are not a Christian); however, this is not based on criteria employed by professional historians but instead carefully crafted methodology that suits their bias--either an anti-supernatural or anti-Christian bias. The astute observer will notice this methodology sets the standard so high that if it were to be applied beyond the Gospels, nothing in the ancient past could be known.
Lastly, it’s one thing if a skeptic can’t take a position on the trilemma because they don’t have enough information. But if they do have the same historical facts of Jesus and the Gospels that we have (which can be easily obtained on the internet and enough of it could be summed up in less than 5 minutes in the video below) then they need to make their choice and be consistent with it. When historical facts are brought up that embarrass the view they hold, they can recant their view and take time to reconsider the options. But they shouldn’t act like a chameleon and change their skin of skepticism mid-conversation for the sake of just trying to poke holes in our worldview. That type of intellectual dishonesty is as blatant as it is repugnant. But, if they are coming from a worldview that requires no moral accountability, why should they be intellectually honest?