18 But deliver us from the evil. If Lead us not into temptation implied the not being tempted at all, He would not have said, But deliver us from the evil. Now evil is our adversary the devil, from whom we pray to be delivered. Then after completing the prayer you say Amen; by this Amen, which means “So be it,” setting your seal to the petitions of the divinely-taught prayer.
Source: Catechetical Lectures (New Advent)