The lost are those who will go into everlasting punishment on Judgment Day (Matthew 25:46). The lost can be split up into these groups.
People who are non-religious. This group has atheists, skeptics, agnostics, etc., and anyone who isn't a member of the Lord's Church (Acts 2:47, John 10:9, Colossians 1:13, John 12:48, 2 Thessalonians 1:7–9, Mark 16:16).
Those in the wrong church. Christ has only one true church and anyone not a member can not be saved (Matthew 16:18, Ephesians 4:4, Eph. 1:22–23, Matt. 15:13–14, 2 John 8–11, Matt. 7:21–27).
Unfaithful members of the true church. These people will be lost because they failed to live true to God's word (Matt. 13:41–42, Hebrews 10:38–39). Check yourself! Are you in one of these groups?