text: FC
  You say...."a priest is a lay person who has received the Sacrament of Holy Orders. "... except that the custom we have developed is that "lay"people are not allowed to 'receive holy orders' and be 'ordained' -- only Clerics. At present 'lay' people are not ordained or allowed to receive Holy Orders.
  The distinction is not between 'lay' and 'ordained' -- it is between 'lay' and 'cleric'.
  
  The Vatican Council documents make it clear that the Bishop is 'The Priest' of the Diocese. Since he cannot be everywhere and do everything, he 'extends his presence' and functions through 'ordination' of person with the power and assignment to 'extend his presence' and perform his ministerial functions of preaching and teaching and presiding at Eucharist, Consecrating, administering Sacraments, Administration, etc.
  
  Dividing Christians into classes of professional 'cleric' and common 'lay' - non-clerics - came rather late. Until then everyone was 'lay' – that's all there was.