"Christian apologetics" is the practice of defending the Bible, and Christian faith and theology, from the criticisms of non-Christians.
The Bible contains many difficult challenges for apologists, but rarely are they as problematic and as densely packed together as in the 7th chapter of the Gospel of Mark.
Mark 7 starts with an argument between Jesus and a group of Pharisees in Jerusalem, on the subject of washing hands before eating. A "tradition of the elders" had become established that said people should clean their "defiled hands" prior to a meal, and the Pharisees objected to Jesus and his disciples failing to do so.
Since that particular "tradition" is mentioned nowhere in scripture, Jesus was incensed, and accused the Pharisees of hypocrisy for establishing "merely human rules" as if they had been ordained by God, and went further to criticize other similarly established human rules that actually went against the commands of scripture, thus "nullify[ing] the Word of God."
So here is the first issue for apologists: despite the insistence of Christian scholars that Old Testament rules are no longer in effect, here is Jesus saying they must be observed, and that to ignore them is "hypocrisy." (This is, of course, not the only time Jesus says something like that.) Apologists would likely easily dispose of this problem, saying something to the effect that in the post-resurrection world, once you "accept Jesus as your savior," then the old laws become unimportant: as Paul said, you are "justified by faith in Christ and not by observing the law." So apologists would regard that one as dealt with.
The bigger problem begins in the next passage: in his very next breath after slamming the Pharisees for not observing Scriptural law, Jesus tosses out the dietary laws. He begins with an argument against the need for hand-washing:
"Again Jesus called the crowd to him, and said, 'Listen to me, everyone, and understand this: Nothing outside you can defile you by going into you. Rather, it is what comes out of you that defiles you.'"
Jesus immediately repeats his argument in the company of the disciples, and Mark then hits the reader over the head with its implications:
"After he had left the crowd and entered the house, his disciples asked him about this parable. 'Are you so dull,' he asked. 'Don't you see that nothing that enters you from outside can defile you? For it doesn't go into your heart but into your stomach, and then out of your body.' (In saying this, Jesus declared all foods clean.)"
Jesus then goes on to elaborate, pointing out that "from within, out of your hearts, come evil thoughts, sexual immorality, theft, murder..." etc., all sorts of bad things, "...all these evils come from inside and defile you."
So there's a lesson about not doing bad things here, but it doesn't erase the damage that's been done in working up to it: Jesus has canceled the Jewish dietary laws, immediately after upholding the sanctity of all scriptural laws -- and not by an argument that "the Covenant has changed and the old Covenant no longer applies," the usual excuse Christians give for ignoring all the many laws given in the Old Testament. Instead, Jesus argues that the dietary laws, given in detail in scripture as the Word of God, never made sense in the first place! One can only imagine God glaring down at his son and exclaiming, "Junior, what are you thinking! You just wait till you get home!" And Mark wants us to be certain that that's exactly what Jesus has done: he adds his own parenthetical comment pointing out that "Jesus declared all foods clean."
On top of this, Jesus has struck a blow against what we now know to be common-sense sanitary practices, the absence of which caused untold suffering through the ages. Some apologists here try to explain that "People back then were too primitive to understand or accept germ theory," but that's completely missing the point: in this passage, Jesus isn't just passing up a chance to promote sanitation, he's actively ridiculing it. He's going to great effort to stop people from exercising a healthy habit they already were practicing!
So all of this, so far, is enough of a problem for apologists, but that's nothing compared with what happens when you figure out why this whole passage is here. By examining Paul's letters to early Christian churches, it becomes clear that "Mark," writing decades after the event supposedly happened, actually invented this entire story for the purpose of settling a debate raging in the early years of Christianity: For the most part, Christians who had been Jews still considered themselves Jews, and constantly argued over whether they, as followers of Christ, were still supposed to keep the original laws, especially the ones about food.
(I will admit that I did not come up with this observation myself. I owe the basic outline of the following to David Fitzgerald, in Jesus: Mything In Action. I have fleshed it out a little.)
The argument over the dietary laws shows up in several of the letters to the early churches from Paul, who was never short of opinions on how Christians were supposed to behave. In Romans 14, for example, Paul specifically addresses the issue of members adhering to different diets, and answers it essentially the same way Jesus speaks on the subject in Mark 7: Paul states that "nothing is unclean in itself," and in particular "all food is clean," so that what you eat or don't eat has no effect on your own salvation, but if you do something that "causes someone else to stumble" (his examples through the chapter make it clear that the "cause of stumbling" here would be the result of telling another person that you don't approve of their diet), then you've done a very bad thing.
The first thing to notice is that Paul passes up a chance to strengthen his argument by quoting Jesus. And in fact, that really is the crux of the problem presented by Mark 7: If Jesus himself made a pronouncement on the issue of which foods to eat and which not to eat by saying that all foods are clean so it doesn't matter what you eat, then why were Christians still arguing about the issue twenty years later? Why didn't Jesus's proclamation settle the argument before it ever got started? Surely if Jesus, the Lord himself, the Christ, makes an announcement of policy, that carries far more weight than anything Paul might say, or Peter, or James, or any other apostle. The answer to any question about the dietary laws should be: Jesus himself said all foods are clean; end of discussion.
If Jesus actually had said that.
The only explanation that makes any sense is that Jesus never said any such thing. If he had, we wouldn't have Romans 14.
Or Galatians 2, in which Paul rips into Peter for Peter's "hypocrisy" in changing his stance on whether the Jewish dietary laws should be observed. Peter had originally been okay with sharing meals with Gentiles, but had decided, after a visit from people sent by James, that the dietary laws must remain in force, so Peter stayed away from eating with Gentiles altogether after that.
The fact that Peter and James both insisted on keeping kosher is especially problematic, because if Jesus had really ever said what Mark claims he said, Peter and James would have been standing right there to hear it. Mark has Jesus saying it to them as they stood among the disciples. Either Peter and James are disregarding a very clear statement they heard from Jesus -- not in the form of a mystifying "parable" that required interpretation, but in words whose meaning Mark considered completely, indisputably obvious -- or else the entire story of what Jesus did and said that day never happened.
Here is the only way I can see of interpreting what Chapter 7 is doing there in the gospel of Mark: That Mark, writing after Paul wrote his letters, took it upon himself to settle the "Do we followers of Christ observe the dietary laws?" controversy by inventing a speech on the subject by Jesus, and a context for the speech: the entire chapter is spun from Mark's imagination as a storyteller. It was done a little inelegantly: As I pointed out, Jesus's argument against the dietary laws implies that God was wrong in giving them, a problem that seems to have escaped Mark's attention, as did the contradiction in Jesus insisting in one sentence that all scriptural laws must be obeyed, and then throwing some of them out in the next.
It goes without saying that as soon as we conclude that Jesus could never have said what Mark claims he said, and that the story is there to satisfy Mark's agenda of settling a dispute among Christians by putting words in Jesus's mouth, that wrecks any justification for believing that anything the gospels claim Jesus did or said really happened: If there are events recorded in the gospels that never happened and quotes of Jesus saying things he never said, how can we rely on any of the Jesus narrative as having really occurred?