Romans 14:4 — Who art thou that judgest another man’s servant? to his own master he standeth or falleth. Yea, he shall be holden up: for God is able to make him stand.

OK. So I can see the claim is a bit extreme. “No bad Christians?” Suppose I said “there are no good Christians”?
It’s just that from time to time I here somebody called a “good” Christian, or I hear of somebody misbehaving that they “are a bad Christian”. Either way, it seems wrong to me.
Remember what Jesus said when the rich young ruler called Him “Good Master”? “And Jesus said unto him, Why callest thou me good? there is none good but one, that is, God.” (Mark 10:18)
I think there are a few things to be remembered.
First, when we come before Jesus for judgment, none of us are righteous … but all of our sins are forgiven. There is a warning, and an assurance in 1 John:

If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. If we say that we have not sinned, we make him a liar, and his word is not in us. (1 John 1:8-10 )

If we will confess our sins we all, equally and with no exception, stand forgiven. If we will not confess, we are probably not saved — not “bad Christians” but “not Christians”!
Second — there are no “better sins” and “worse sins”. We have a tendency to rank sins, as though his adultery was not so bad as her lesbian lifestyle, or her theft was worse than his bullying. There is no “league table”! All sins separate us from God — and for a Christian, all sins can be forgiven. I know this seems extreme — surely stealing a loaf of bread isn’t the same as genocide. That’s fair — and Jesus certainly said that a servant knowing His will and not doing it would be beaten more than someone who strayed unknowingly — but it seems to me that the cases are not the same. The servant adds disobedience to his other faults …
The last though I had is the one that flows from Paul’s comment in the letter to the Romans. Only God can judge Christians. Nobody else can call a Christian “good” or “bad”. Only God sees the heart … only God knows why we do what we do.
There are no bad Christians. Only Christians who haven’t finished growing!