I believe that there is a certain amount of due diligence necessary when a person, a couple, or a family move in to the area and want to become members of a local congregation of the Lord's church. What do you recommend? What do you do?
One scripture that comes to mind is II John 9-11: Whoever transgresses and does not abide in the doctrine of Christ does not have God. He who abides in the doctrine of Christ has both the Father and the Son. If anyone comes to you and does not bring this doctrine, do not receive him into your house nor greet him; for he who greets him shares in his evil deeds.
We would not want to share in someone's evil deeds.
It's not that we need to be nosy, but we do have to be certain that we are not allowing a "little leaven into the lump." Any thoughts on this subject would be appreciated.
Please notice that in both II John and in I Corinthians 5 sin was noticeable. In I Corinthians 5, a man had his father's wife. People knew it was happening, but they did nothing. It was their lack of response to sin for which Paul scolded them, not their inability to anticipate that the sin would occur. In II John 9-11, the problem was someone coming and not teaching true doctrine. It wasn't a question that false doctrine might be taught, it was not accepting false doctrine and its teachers when it is taught.
It rarely takes long to figure out where a person stands on doctrine. One of the earliest questions brethren often ask is where a person is from and if they know some of the same people. On the rare occasions when something is said that makes me wonder, I'll send a note or call up people mentioned to ask if they know the person in question. I rarely just take someone else's word for what a person is said to believe, I'm looking for areas that need to be addressed. I then ask the person about those issues.
We don't ask a person to teach a class or present lessons until we get a feel for what they believe. "Do not lay hands on anyone hastily, nor share in other people's sins; keep yourself pure" (I Timothy 5:22). If something is taught wrongly, we deal with that issue first before asking them to teach again. We've had people come in holding false ideas, but with study and time we have persuaded them to follow after Christ and they became solid members of the church. Others end up leaving because they would not give up their false ideas.
For example, we had one young man come with what appeared to be a wild tale of misdoings where he came from. Things didn't add up in his story, but after some research it was decided that there was a remote chance that he was right. We told him we would not accept him as a member until six months went by, and he agreed. In less than two months, after being caught several times lying, he left. Throughout the time we worked hard to get him to repent of his sins. If he had changed, we would have accepted him. But it didn't happen. His actions verified that his tale was a lie. "They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us" (I John 2:19).