He usually made good profits of the shrines, but since Paul had started preaching his profits had gone down. Since Demetrius was so concerned about this, he gathered his buddies together and told them how it was. The temple was a sacred worship center for the goddess Artemis. The craftsmen were outraged by this and starting yelling and shouting out chants to Artemis. It all comes down to this- that serving other gods is wrong and only causes problems.
Money is the root of all evil.
Put no other gods before God. People that live in the world are truly from the world.