I am not talking about unforgivable sin, I want to know what sins a saved person might commit that would cause you to doubt that they were ever saved in the first place.
We can't be certain:
When someone says they're a Christian, and they don't seem to act like it, no one can ever be 100% certain about the condition of their soul.
In the Bible, we only seem to find certainty about the condition of our OWN soul.
However, even though we cannot be certain of others, we can still be wise, and look for patterns of behavior that would seem to indicate an underlying condition.
Patterns to Determine an Underlying Condition:
This is the same way that we'd look for patterns to determine physical illness, or mental disturbances. If I cough once or twice, it might mean nothing. But If I cough all day and night, that's a pattern of some underlying condition. If someone tells a lie once or twice, we might not think too much of it. But if everything a person says is always a lie, we know that's a pattern of an underlying condition.
There are also patterns we can apply to spiritual things, to make an educated guess about someone's spiritual condition.
1. The Bible says Christians are God's children, and like children, he chastens us when we go astray.
So if we see someone claiming to be a Christian, and they've been committing a great deal of sin, for long time, and there's no chastening, and everything in their life is fantastic... that's a sign they may not really be a Christian.
2. The Bible says God promises to work in us and change us, and conform us to his image.
So if we see someone claiming to be a Christian, and they haven't made any changes in their life at all since they converted, and they live just the same as they ever did, and they have no interest in spiritual things... that's a sign they may not really be a Christian.
3. The Bible says God puts the holy spirit in us to guide us, and he convicts us of sin.
So if we see someone claiming to be a Christian, and they've been committing a great deal of sin, for a long time, and they don't seem to show any signs of guilt or remorse for anything they've done... that's a sign they may not really be a Christian.
Conclusion:
These are some of the basic things we might look for, to make an educated guess.
So to make that educated guess, we would not look for a particular sin, but we would look for a long ongoing pattern, a pattern that would be contrary to what SHOULD be going on in a Christian's life. When the pattern is long and ongoing, it is probably a good indication of an underlying condition.
So we can never be certain about the condition of someone claiming to be a Christian, but looking for ongoing patterns can help us to make a good educated guess.