The Greek noun theiotēs refers to the divine nature, quality, or character — what it means for something to be divine. It appears only once in the New Testament (Romans 1:20) and is distinct from the closely related theotēs (G2320, Godhead/Deity in the fullest ontological sense). Theiotēs refers to the quality of divinity — the attributes and power that characterize God — while theotēs refers to the essence of being God. Both are important in different theological contexts.
Paul uses theiotēs to argue that God's divine nature is clearly perceived in creation — this is the foundation of natural theology. The invisible God has made Himself visible through the things He has made. Therefore, all people have sufficient knowledge of God to be accountable (Romans 1:20–21). This does not save them — but it leaves them without excuse. Only in Christ is the fullness of the Godhead (theotēs, Colossians 2:9) dwelling bodily. Creation reveals God's power and character; Christ reveals His Person and heart. The two go together: The heavens declare the glory of God (Psalm 19:1), and He is the radiance of God's glory (Hebrews 1:3).