Katallassō (G2644) means to reconcile, to change from enmity to friendship. The prefix kata- intensifies the root allassō (to change/exchange), giving the sense of a thorough, decisive change in relationship. The noun form is katallagē (G2643, 'reconciliation'). It appears 6 times in the New Testament (4 occurrences in Romans and 2 Corinthians), yet it is theologically indispensable. A similar compound, apokatallassō (G604), appears in Ephesians and Colossians.
Reconciliation is one of the great Pauline metaphors for the atonement, and katallassō is its verbal core. The starting point is enmity: humanity is 'hostile to God' (Romans 5:10; 8:7; Colossians 1:21). This is not merely estrangement but active opposition — 'enemies' is the word Paul uses. The cross does not merely improve a relationship; it abolishes enmity and creates peace where war existed.
Critically, Paul says God reconciles — not that humans reconcile themselves to God. '...God, who through Christ reconciled us to himself' (2 Corinthians 5:18). The initiative is entirely divine. Yet the appeal is real: 'We implore you on behalf of Christ, be reconciled to God' (2 Corinthians 5:20). God has done the work; humans must receive the reconciliation. The ministry of the church is thus a 'ministry of reconciliation' — ambassadors announcing the peace treaty God has already signed in Christ's blood.