You gotta admit it's a lot to know off the top of your head. For example, I can't fully explain every Catholic belief off the top of my head -- I could probably do okay for a lot of things, but for a full and inerrant explanation, wow, that would require memorizing hundreds of thousands of words and likely getting some kind of degree in Catholic theology. (I can't recite the full text of the Bible either, believe it or not.) But I'm familiar with the beliefs and any beliefs that didn't seem obvious or logical to me I've looked into more closely to the point where I accept them as truth.
It's similar to mathematics, which I've also studied. Just because I can't reproduce a particular proof off the top of my head doesn't mean the theorem isn't legit. You study it, accept it, and move on.
Ideally we're supposed to be able to explain everything in order to evangelize. But most fall short of the ideal in many ways. And there is something to be said for coming to Christ with the faith of a child -- accepting and believing.
Agreed, but considering how large my wife's family is, and the fact that I know several others who are Catholic, I was surprised at how little they did know. I don't claim to be anywhere near an expert on the Catachism (and I'm not Catholic), but I have a better understanding than many of my Catholic relatives. And any of then don't agree with everything that is inthe Catachism. I have trouble understanding how one can claim to be a member of any denomination if they don't adhere to the basic beliefs of that faith. It is almost like some of them look upon the Catholic church as a social club, or as something that they are a member of because their parents and grandparent were members. I don't agree with some of the things, but I at least understand what the thought process is. (I have the same problem with some friends of mine who are very conservative when you talk with them about politics, yet always blindly vote democratic. Why? Because their parents and grandparents were democrats, so they vote that way because that is just how they are, and are really unaware that the candidate they voted for holds the opposite of their personal views).
Ideally we're supposed to be able to explain everything in order to evangelize...
I agree to a point. The more you know, the more confidence one can have when sharing the gospel. Sometimes though knowledge can be a hindrance. Trying to answer every question someone has can lead one on "rabbit trails" attempting to lead the conversation away from the Gospel. The blind man who had his sight restore is one of the strongest witnesses to come before the Pharisees. And he admitted he didn't know the answers to all their questions, yet he made a bold and wonderful witness. The madman healed by Jesus desired to follow Jesus, yet Jesus told him to return to his home and share his experience with others. The healed madman didn't undergo vast training to answer questions. He was just told to go and tell others.
I myself have recently completed a training course in evangelism. One of the main things we are taught is not to allow our bible knowledge to hinder us. To use discretion when sharing the Gospel. When witnessing, if a question asked will help in leading one to Christ, we are told to go ahead and answer it, but if the person asking the question it doing so in a way to divert the message, we should keep the "main" thing the "main" thing: Keep to the sharing of the Gospel and make the other questions secondary... something that you can use to followup with. A Person you are sharing the Gospel with might ask a question that would lead you away from the topic. I will tell them "Hey, that's a good question. Let me research it and get back to you with an answer". If they are truly interested, they will allow you a means to contact them, and it gives you a chance to contact that person again to further witness.
We need to keep to the essential message -
These are not easy things for us to do, but going to the Cross was not and easy thing for HIM to do either. We need to love the Lord as he loves us.