Consider baptism, although the same discussion applies to membership and communion.
You know how there are two types of people in the world, those who like Neil Diamond and those who don't? Well there are also two types when it comes to these sorts of questions:
Type 1: Those who act as if, when they meet God, they fear the question: Why did you baptize X? more than the question Why didn't you baptize X?
Type 2: Those who act as if, when they meet God, they fear the question: Why didn't you baptize X? more than the question Why did you baptize X?
I don't know what's right here, but I was definitely Type 2. I would rather have erred on the side of baptizing by mistake rather than, though my very fallible discernment, refusing to baptize a believer.
I would base that on the following:
- Baptisms in the New Testament appear to be almost immediate. There was no time allotted to probe the authenticity of a testimony or question the person on any finer points of theology. It was believe and be baptized. As in, right now, or maybe, at most, we wait to the morning.
- When it was a mistake, as in Simon the magician, there was no weeping and gnashing of teeth. That was a perfect teachable moment to explain why we need to be very careful whom we baptize. Instead it was like, "meh--toss the bum out and let's move on."
- My reading of scriptures pertaining to the sacraments and membership leads me to believe the onus is on the person, not the or pastor, to partake in a worthy manner. Except for case where I know the person to be in unrepentant sin, I am not concerned about giving communion to someone who shouldn't have it--they should be concerned about that.
Having said all that I am sympathetic to those Type 1 folk who take a more cautious approach. I don't agree, but it's entirely possibly that they are right and I am wrong.