Opus Dei is an organization for laity. They do however have an order of priests to tend to the needs of the people… Dominicans were founded by St Dominic, they are the Order of Preachers, known for sound doctrine and great speaking skills. Jesuits were founded by St Ignatious Loyola, they were the “shock troops” of the Counter-Reformation, in complete solidarity with the Pope. They too are known for sound doctrine, though in recent years some weird ones have popped up. Basically every order was founded by a great man/woman who felt God was calling them to do something special (St Francis to reform and rebuild the Church) thus they gathered like minded people to themselves.
