From 1st Cor. 1:
1Brothers, I could not address you as spiritual but as worldly—mere infants in Christ. 2I gave you milk, not solid food, for you were not yet ready for it. Indeed, you are still not ready. 3You are still worldly. For since there is jealousy and quarreling among you, are you not worldly? Are you not acting like mere men? 4For when one says, "I follow Paul," and another, "I follow Apollos," are you not mere men?
Sounds like Paul is definitely addressing saved folks here. He calls these people "brothers" and "infants in Christ", not "infants out of Christ". He then goes on to accuse them of quarreling and claiming to follow different men, the cause of the divisions.
So, at least some of the trouble makers were christians.
But, I still don't see anything that demands exclusion from a 1000 year earthly kingdom.
Les