The Greek pronoun sy (σύ) is the second-person singular personal pronoun, meaning 'you' (as in one individual person). In Greek, the pronoun is often embedded in verb endings, so when sy appears as a separate word, it is typically emphatic — 'YOU, specifically.' This emphasis makes many New Testament uses of sy theologically significant.
The emphatic use of sy creates some of Scripture's most powerful personal confrontations. When Pilate asks Jesus, 'Are you the king of the Jews?' (Matthew 27:11), the emphatic pronoun challenges Jesus to self-identify. Jesus uses the emphatic sy when telling Peter, 'You are Peter, and on this rock I will build my church' (Matthew 16:18). Most tellingly, when Jesus asks the disciples 'But who do you say I am?' (Matthew 16:15), sy shifts the question from abstract discussion to personal commitment. The gospel always makes this move — from 'what does everyone say?' to 'but what do you say?' Every person must answer for themselves.