Jewish monotheism makes the charge of making oneself God especially serious.
Deuteronomy 6:4 provides the broad monotheistic background.
The opponents' charge is not sufficient by itself, but Jesus does not retract the unity claim. He proceeds to defend his unique consecration and mutual indwelling with the Father.
The passage explicitly places the question of Jesus' deity within the narrative and connects it to his unity with the Father.
The opponents pick up stones and accuse Jesus of blasphemy.