Believest thou not that I am in the Father, and the Father in me? the words that I speak unto you I speak not of myself: but the Father that dwelleth in me, he doeth the works. (John 14:10)
Jesus had earlier stressed His unity with the Father (see on ch. 10:30).
The words.
The words and works of Jesus both bore testimony to His divinity. The disciples should have believed the word of Jesus. If this was difficult for them, they should have accepted His word on the basis of His works.