Moses and prophets could reveal God, but none normally claims that seeing him is seeing God in this comprehensive sense.
Exodus 33:18-23 provides a striking contrast with the Old Testament desire to see God.
Jesus grounds the claim in mutual indwelling and in the Father's words and works being present in him.
This is a central Johannine divine-presence text: the unseen Father is uniquely and personally revealed in Jesus.
Philip asks Jesus to show the Father.