Exaggello (G1804) combines ex (out, fully) with aggello (announce, proclaim) for an intensified declaration — not merely telling, but publishing broadly and completely. Used only once in the NT (1 Pet 2:9), it describes the purpose for which believers have been made God's 'royal priesthood and holy nation': to 'proclaim [exaggello] the excellencies of him who called you out of darkness into his marvelous light.'
The theology of exaggello centers on the missional identity of the Church. Peter draws on Exodus 19:5-6 (God's call of Israel) and applies it to the new covenant people: just as Israel was called to be a kingdom of priests mediating God's presence to the nations, so the Church is called to actively declare — exaggello, proclaim fully and broadly — the great things God has done. Salvation is not merely received; it demands proclamation. The transformation from darkness to light (Col 1:13) is too glorious to remain private.