Joshua 1:9 — "Be strong and courageous. Do not be afraid; do not be discouraged, for the Lord your God will be with you wherever you go" — was spoken to Joshua as he prepared to lead Israel into the Promised Land after Moses' death. It's a command paired with a promise: courage isn't required to go it alone, because God's presence goes with it.
It's a natural fit for apparel meant to encourage someone facing something big — a new season, a hard decision, a fight worth having.