What is so remarkable about Simon being astonished?
Acts 8:13 (ESV)
13Even Simon himself believed, and after being baptized he continued with Philip.And seeing signs and great miracles performed, he was amazed.
Previously, the Samaritans had been amazed by Simon’s work of magic, but now it is Simon himself who is astonished as he follows Philip around and sees the signs and great miracles being done. This represents a complete turning of the tables.
13 Even Simon himself believed, and after being baptized he continued with Philip. And seeing signs and great miracles performed, he was amazed.