Asphalos (ἀσφαλῶς) is an adverb meaning "safely," "securely," "certainly," or "beyond all doubt." It derives from asphalos (safe, firm, certain), itself from alpha-privative and sphallo (to stumble, to trip). The word appears in Acts 2:36 (Peter's Pentecost sermon), Acts 16:23 (the jailer's instructions), and Mark 14:44 (Judas's betrayal signal).
The three New Testament uses of asphalos span the full dramatic arc of the gospel. In Acts 2:36, Peter declares with certainty: "God has made this Jesus, whom you crucified, both Lord and Messiah." In Mark 14:44, Judas instructs the guards to arrest Jesus and lead him away securely. In Acts 16:23, the jailer is told to guard Paul and Silas safely — the same jailer who, by midnight, would be asking how to be saved. The word that meant "hold him securely" in earthly terms was subverted by divine power. The gospel has a way of turning every attempt at secure containment into an opportunity for liberation. What appeared "safely" guarded became safely opened by the earthquake of God's sovereign power.