Biblically, there are legitimate and illegitimate reasons for divisions/separations. God causes some divisions and He calls His people to do the same.

Sin causes universal separation from God (Isa. 59:2; Rom. 3:23).

Fleshly quarrels cause divisions (1 Cor. 1:10-13); though such quarrels are necessary to distinguish the spiritual from the immature (1 Cor. 11:18-19).

Unbelievers cause division in the church (Jude 1:17-19).

God divided rebellious men at the Tower of Babel (Gen. 11:1-9).

God separated Israel out from Egypt during the exodus (Ex. 3:1-14:31).

Jesus’ teaching caused division (Luke 12:51-53; John 6:66; 7:40-43; 9:16; 10:14-19).

God separates believers from Satan’s kingdom of darkness and transfers them to His kingdom (Col. 1:13-14).

Christians should not be bound with unbelievers (2 Cor. 6:14-17).

Believers should separate from false teachers (Rom. 16:17; 2 John 1:7-11), and those who live in gross immorality (1 Cor. 5:1-2, 9-11; 2 Tim. 3:1-5).

Jesus will separate people at His Second Coming (Matt. 25:31-33).

Jesus will separate unbelievers to the Lake of Fire (Rev. 20:11-15).