The Greek form ei is the second person singular present indicative of eimi ('to be') — meaning 'you are.' Though a grammatical form, its occurrences in Scripture are among the most theologically loaded utterances in the New Testament, particularly in confessional and identity declarations addressed to Jesus.
Three confessional uses of ei stand above all others. Peter's confession: 'You are (ei) the Christ, the Son of the living God' (Matthew 16:16) — to which Jesus responds that flesh and blood did not reveal this. Jesus's question to the high priest elicits: 'You are the Christ, the Son of the Blessed' (Mark 14:61) — to which Jesus answers 'I am' (ego eimi). The Father's voice at the Transfiguration and Baptism ('This is my beloved Son') parallels what Peter and others confess with ei. These second-person declarations of identity are the hinge points of Christological recognition in the Gospels.