This question doesn't really make any sense when you think about it. First, you need to define a Christian. Since the only universally accepted definition of a Christian is someone who believes in Christ, it would appear that it is really easy to be a Christian. However, there are so many lairs to this - reading the Bible, attending church services, praying and other activities could make it make it significantly more difficult to be a Christian. Since all religions allow its believers to practice their religion the way they want, and if they don't it makes them a cult, it is up to each individual to decide how actively religious they want to be.
However, with that being said, it could be viewed extremely difficult to be a Christian. Why? Because if you studied it and came to the conclusion that it wasn't true, then you cannot be a Christian, from the universally accepted definition of it. The beliefs that come in form of creeds are the reason why people actively participate in their religions. With the advent of creedless religions such as Unitarian Universalism that have developed in the 20th century, it could be perceived that those religions are easier to follow because there is a lack of self-denial during events. However, all creedless religions have some sort of glue of beliefs that hold the religion together. For the UUs it's political progressivism.
Ultimately, the easiest religion to be is the one you already believe in, no matter how devoted you get in that religion. It's harder to participate in a religion casually if you partaking in self-denial than it is if you are frivolously religious but truly believe the overall message of the religion. Therefore, this question is too subjective to answer. I myself have decided to stop attending Mennonite services due to the self-denial during service and Sunday worship. It was too difficult for me to practice the religion due to that. I'm going to attend Unity services soon and I'll let RF know how that ends up. I'll probably post an update on my thread Brain Droppings about it.