Heis is the Greek numeral 'one,' including its forms mia (feminine) and hen (neuter). It is one of the most theologically loaded words in the New Testament. It expresses monotheism ('the Lord our God, the Lord is one' — Mark 12:29), the unity of the Godhead and the church, the uniqueness of Christ's mediation ('one mediator' — 1 Timothy 2:5), and the completeness of the new creation.
The Shema's declaration 'The Lord is One' (Deuteronomy 6:4) is the cornerstone of Jewish and Christian monotheism. Jesus affirms this in Mark 12:29, then adds the second commandment, linking love for God and neighbor. The New Testament extends heis theology into Christology ('one Lord' — 1 Corinthians 8:6), ecclesiology ('one body, one Spirit' — Ephesians 4:4-6), and soteriology ('one mediator' — 1 Timothy 2:5). The great prayer of John 17 — 'that they may be one (hen) as we are one' — grounds the church's unity in the intra-Trinitarian unity. All divisions in the church are thus not merely organizational problems but theological contradictions.