This is anyone who is not a believer in Jesus Christ, and if they die without accepting His gift of salvation they will be doomed forever to the lake of fire, commonly called hell (Rev 20:15, 21:8). This includes those who have ‘a form of religion’ yet do not have a vital personal relationship with Him, besides those entrapped in false religions or belief systems.
Are you a non-Christian or Christian?
Bible especially the NT. Thus, a non-Christian person or non-Christian religion fails to agree with, and follow the essential, non-negotiable ‘facts’ of Christianity.
We are to relate and associate with those who don’t know Christ, with the hope they will become followers of Christ, however believers are not to enter into binding agreements (marriage and business partnerships) with those who are not Christians (2 Cor 6:14-16). While we are to be separate from the non-Christian’s sin, we are not to isolate ourselves from them as people, rather being witnesses to them of Jesus who loves and died for them.