The tension between law and grace, faith and works.
I've struggled with this one before, and have written on it here. One has to shed a lot of what Luther read into the text in order to come to terms with the seemingly contradictory statements regarding Christian life in Scripture. Christianity is about living the kingdom of God, embodying the reality that Jesus accomplished on the Cross. Thus, works consist of our active participation in the kingdom of God, in Christ's reality. The law is not made up of rules, but ways we can apply the kingdom to our lives. It is easy to turn this into legalism, but that's when we start regarding Scripture as a rule-book.It's hard to deal with/love fundamentalists and/or people who believe and teach damaging doctrines.
How patriarchal the Bible mostly is.
One major consideration needs to be made when dealing with this issue: context. It also needs to be addressed on a text-by-text basis, and often simply considering the historical context will remedy the problem. There are so many good books on this topic. Why Not Women? is one that was highly recommended to me, but there are many others. That would be a good place to start. For an assessment of one text, see my, "Does Genesis 2 Support Complementarianism?" (here). It should be said, though, that the "patriarchal nature" of Scripture is often over-stated. In many places, the Bible is quite woman-exalting.Most Christians are so certain of what that they believe that they never question it. A great many Christians do not engage in critical thinking.
Christian hypocrisy and self-righteousness, manifested in things like treatment of/talk about homosexuals, or people of other religions.
What about those who have never heard the Gospel?
I can see this one being very hard for Calvinists, but it isn't hard for me at all. While it is unfortunate that so many people in history, as well as in our time, are not subjected to the Gospel, if one affirms a great wideness in God's merciful saving love, we can feel at peace about this issue. I have written about this here. A good book to look at on this is A Wideness in God's Mercy by Clark Pinnock, as well as those cited in my blog on the subject."I know my level of intelligence and understanding and there are a lot of better equipped thinkers out there who do not believe what I believe. So I wonder, who am I to think this stuff is true?"
Prayer. How come some prayers are answered but most aren't? How come someone's headache is healed, but someone else's cancer isn't? Don't we serve an all-powerful, all-loving God? Why wouldn't he heal everyone if he could?
I have some pretty bad back and neck problems that I wake up with every single morning, so this was something I struggled with for a while (and still do, depending on how bad I feel on a particular day). I have written on the subject here, "Petitionary Prayer: What is it Good for?", but I also recommend Greg Boyd's book Is God to Blame?, as well as his chapter, "Praying in the Whirlwind" in Satan and the Problem of Evil. Vincent Brummer's What Are We Doing When We Pray? is another good one. There is a lot of really good literature on this topic, but those are good places to start, as well as the works cited in my blog.The problem of evil and suffering. It is difficult to reconcile God's benevolence with the existence of things like cancer and AIDS.
All honest Christians struggle with this at one point or another. While I have read a lot on the subject, I can think of no better person to go to than Greg Boyd. Specifically, his book Satan & The Problem of Evil. If you're really into the topic, supplement it with his God at War. For a shorter, easier read, see Is God to Blame? There are tons and tons of books on this topic, but Boyd would be the perfect person with whom to start.Pluralism. If I were born in other parts of the world, I would not be a Christian. Do I think Christianity is true simply because of my context? How can I be so bold as to claim that the religion I follow is THE truth? There are thousands of religions all over the world, and plenty have their holy books, what makes Christianity so special?
This is an issue that is often daunting to me, and very humbling when I speak of things that I 'know'. For me, this is where the rubber meets the road, faith-wise. I wrote a personal reflection on pluralism here, saying that, in the end, one has to look at the subjective truth claims of this world and ask oneself, "Which one is more worthy of my faith?" because we can't know that Christianity is actually true.Angels and Demons, Heaven and Hell. They seem so make-believe, as if they were made up to deal with death.
This is a tough one. What makes it worse is how angels, demons, heaven and hell have been portrayed over the years. We tend to have a shared concept of angels as winged, effeminate and covered in white; demons as ork-like or dragon-like beasts; heaven as a golden city in the sky; and hell as a fiery pit with black castle-like buildings, where people are tortured by demons. I have tried to re-word these terms in an effort to steer away from these preconceived ideas--using terms like "metaphysical" instead of supernatural, forces of chaos, instead of demons, and others. Sometimes, when words have been ascribed images that make them ridiculous, we just have to come up with new ways of referring to them.
Another thing that helps me is to recognize the signs of their existence in the world. We have desires that go beyond our experience, that go beyond what we can satisfy. We have urges to do horrible things sometimes, and we often don't know why we do such things. Sometimes things happen that can only be sufficiently explained by divine hands (I find all other "natural" explanations dissatisfying). Other things can only be explained by an actual force of evil at work--attributing the misfortunes in nature to the trial-and-error process of evolution isn't going to do it for me. I can think of many examples, but you get where I am going. There are signs in the world around us that point us beyond, and affirm what we see in Scripture.