The KJV does not say "God" in Acts 7:20 because θεω in the context is to be understood as a Hebraism. Strong's Hebrew and Greek Dictionary says concerning "θεος":
Of uncertain affinity; a deity, especially (with G3588) the supreme Divinity; figuratively a magistrate; by Hebraism very: - X exceeding, God, god [-ly, -ward].For example, the Hebrew word for God, "אלהים" is translated "exceeding(ly)" in Jonah 3:3 in many translations (e.g. KJV, NASB, ESV, NRSV, NKJV). The context in Jonah 3:3 is clear that Ninevah was an "exceedingly" large city ("of three day's journey"), not a city of God (the city was steeped in sin) or a city considered large by God (all man-made cities are minuscule in God's sight). The Septuagint, carrying Hebraisms over into Greek, translates this portion in Jonah 3:3 as "μεγαλη τω θεω" (Lit. "large to God"), which idiomatically means "exceedingly great/large." Likewise Luke, being familiar with Hebraisms, used a Hebraic expression in Acts 7:20 even though he was writing in Greek. Just as "τω θεω" in the Septuagint Greek text of Jonah 3:3 means "exceeding," "τω θεω" in the Hebraic Koine Greek text of Acts 7:20 also means "exceeding."
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