The Greek infinitive einai is the present infinitive of eimi — 'to be.' While it seems grammatically basic, its theological uses in the New Testament are profound, particularly in the absolute declarations of Jesus ('I AM') and in Paul's explanations of what it means to 'be in Christ.'
Paul's phrase 'to be in Christ' (einai en Christō) is the fundamental description of the believer's existence. This is not metaphorical but ontological — a real transfer of identity, status, and life. The absolute use of einai in Philippians 2:6 ('who, though he was in the form of God, did not count equality with God a thing to be grasped') describes the pre-existent divine being of the Son. Einai bears the weight of trinitarian theology and the mystery of incarnation.