“Do not let your hearts be troubled. Have confidence (believe) in God, have confidence also in me.”
Conscious of their troubled thoughts and hearts He set out to encourage them. And He did it by pointing out what He would be doing for them in the future. Let them not be afraid. They must not waver in their confidence. They not only have to hold to their belief in God, they have to hold to their belief in Him. This indeed is to be their rock and their confidence, that, whatever happens, they continue to recognise in Him the One Who has come from the Father, the One Who reveals the Father, the One Who brings men to the Father, the Expected One. That is where their confidence must now lie.
Whether the verb is indicative (‘you believe') or imperative (‘Have confidence!') matters little. Either translation of the word is strictly correct, but the meaning is the same. It is an encouragement not only to maintain their confidence in God, but also to have the same confidence in Jesus. It is a claim to equality with God.
The word ‘heart' is in the singular, ‘the heart of you all.' It may mean ‘each of your hearts' or ‘all your hearts' seen as one in a collective noun.