The Greek verb existemi (ἐξίστημι) means to astonish utterly or to be beside oneself. Literally it means to displace from a standing position — to knock someone out of their normal mental equilibrium. It appears 17 times in the New Testament and is consistently the response to Jesus' miracles and the Holy Spirit.
Existemi is the vocabulary of theophany. Crowds were astonished at Jesus' teaching and miracles. At Pentecost, the crowds were amazed and perplexed. The disciples were utterly astounded at the feeding of the 5,000. A Christianity that is never astonished has lost contact with the living God — the proper response to divine in-breaking is always existemi.