An analogy related to the post a couple of days ago regarding the need to live the Christian life:
I am a mailman, a postman, a letter carrier. However you say it, that is what I am.
But what makes me a mailman? How do I know that I am a mailman?
Yes, the post office hired me. I applied, took a test, waited for months and then they called. They hired me. But that in itself did not yet make me a mailman.
I went to training. But that did not quite make me a mailman.
I went to work, was given a spot in which to work and shown what to do. But I wasn't a mailman yet.
I began to sort the mail. I prepared the mail and took it to the street. Not a mailman yet.
I put mail into people's mailboxes, checking the names and addresses carefully to make sure it was the right mail in the right box.
I was a mailman! Suddenly it wasn't theory, it wasn't a possibility, it wasn't a goal, and it wasn't dubious. It was certain! I was a mailman because I was doing what mailmen do.
Yet I didn't make myself a mailman. I was called to it. Actually I was called via a postcard in the mail that told me to report to training. No amount of walking down the street fiddling with other people's mailboxes would have ever made me a mailman if the US Postal Service had not chosen and called me.
Conversely, (some extra sensitive Calvinists won't like this part) the US Postal Service did not make me a mailman either. In one sense they did. They called, hired, and trained me; they gave me uniforms, a title, and a place to work. But until I began, rather slowly and hesitantly at first, to actually deliver mail I was not a mailman.
I know that I am a mailman because I live the life of a mailman. Similarly I know that I am a Christian because I live the life of a Christian. (The question of how to define what that life is does not concern me here, just that there is such a life.)
Now as a mailman I do not know everything. I do not really know who lives in the houses. I know only who the tenants tell me lives there, if I can get them to tell me. On my mail route there are very many college students. They tend to move rather frequently, often three or four times a year. And they don't always let me know. So, as I deliver the mail I am very careful to always put only the mail that belongs to a house in their mailbox. I often knock on doors to ask exactly who should currently be getting mail at that address. I am careful.
But no amount of being careful could ever make me one hundred percent accurate. These students move far too often. I know as I am delivering my mail that there are likely to be some homes to which I am delivering mail that no longer belongs there. I am sure to make mistakes that are purely my own fault too. But I am still a mailman, even if I can't be a faultless mailman. I am a mailman because I deliver the mail, not because I am never wrong in some deliveries.
However, lest you hear me saying that it does not matter whether the mail goes to the right home or not, let me assure that it matters very much. I would not be a mailman for long if I didn't care whether the deliveries were accurate. In fact if I really didn't care you might say that I was not even a mailman at all, just some guy who wore a uniform that didn't rightfully belong to me and who was embarrassing the postal service.
Likewise, people who are living the Christian life will not be right in all doctrine, they will not be perfect in all matters of conduct, and their worship style may fall short of the angels saying "Holy, holy, holy," before the throne in heaven. Yet we are truly and exuberantly living the Christian life. We are LIVING, despite our shortcomings.
These shortcomings do not stop us from living the Christian life. Not at all. But they matter. They matter to God, perhaps. But they also matter to the people engaged in this life that Jesus has given us. We care! It is not that by getting it right we will suddenly become true Christians. We already were true Christians. It is that if we didn't care whether our understanding, actions, and worship were appropriate, that would just be proof that we really aren't living the Christian life. We would be disclosed as those who are merely wearing a uniform that doesn't really belong to us.