The Greek word endeixis means proof, demonstration, or evidence — the concrete showing forth of something. In Paul's letters, it appears in passages about the demonstration of God's justice and righteousness.
Romans 3:25–26 uses endeixis twice in a pivotal theological passage: God put forward Christ 'as a propitiation by his blood... This was to show (endeixis) his righteousness... It was to show his righteousness at the present time, so that he might be just and the justifier of the one who has faith in Jesus.' The cross is God's supreme endeixis — His definitive proof that He is both righteous (He does not overlook sin) and merciful (He provides the sacrifice Himself). Philippians 1:28 uses it for the believer's courageous conduct as 'a clear sign to them of their destruction, but of your salvation, and that from God.' The life of faith is itself a demonstration — an evidential display of the gospel's reality.