Actually yes there is
1. They left us, You can't leave us unless you have been walking with us. No one does that unless the claim to be a believer,, No person who is an unbeliever would be considered to be walking with us
2. they were not of us - They were not a true part of the church. they just walked with us. not everyone who walks with us in the church is saved. there are many make believers who never had true faith, then left and now deny christ. they are anti christ.