Theos — God
אֱלֹהִים
God — the Greek Scriptures' rendering of Elohim, carrying the whole Tanakh's freight into the NT.
Every NT Theos stands on Elohim's shoulders through the Septuagint. The word itself was ordinary Greek; the Gospel filled it with the God of Abraham — and John 1:1 stretched it to hold the Word made flesh.
First occurrence
Matt 1:23 — 'They shall call His name Immanuel — God with us'
Key passages
John 1:1 — 'The Word was with God, and the Word was God'
John 3:16 — 'For God so loved the world'
Rom 8:31 — 'If God is for us, who can be against us?'
In the Greek Scriptures
Θεός