From Latin simplicitas ("singleness, plainness, uncomplicated"). In biblical ethics, simplicity is the freedom of a life uncluttered by possessions, ambitions, and hidden motives; the wholeness of one who wills one thing, owns few things, and says what he means. Greek haplotēs — "singleness, sincerity" (Romans 12:8, 2 Corinthians 9:11), from haplous ("single, without folds"). The word-picture is of an unfolded cloth — one piece, not layered with concealing complications.
Simplicity is freedom. The person who wants little can go anywhere, tell the truth at any cost, and walk away from any idol. The person entangled in debt, hoarded stuff, constant purchases, and the anxious management of an inflated lifestyle is a servant to his possessions. Jesus: "Do not lay up for yourselves treasures on earth" (Matthew 6:19-21). Paul: "I have learned, in whatsoever state I am, therewith to be content" (Philippians 4:11). John the Baptist lived in camel's hair; Jesus had nowhere to lay His head; Paul made tents; the prophets lived light. Richard Foster's Celebration of Discipline makes the classic modern case that simplicity is an inward focus (seeking first the kingdom) that produces an outward lifestyle (contentment with enough). Practices: buy things for their usefulness, not their status; give more than you keep; avoid debt; resist the tyranny of appearances; speak plainly (let your "yes" be yes); refuse what enslaves. Modern consumer culture is a discipleship program in the opposite direction — every ad is a catechesis that says you are what you own. Simplicity deprograms you.