Believed is the main verb in the sentence and connected to it are two participles. Most translators apparently understand the first participle in a temporal sense, when he saw what had happened (but see Jerusalem Bible, which takes it as a descriptive participle, “who had watched everything”). It seems better to take the second participle in a purely descriptive sense, he was greatly amazed at the teaching about the Lord, rather than to take it in a causal sense as though this was the basis for the governor’s faith (see Jerusalem Bible “became a believer, being astonished by what he had learnt about the Lord”).

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on The Acts of the Apostles. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .