To receive real benefit from the performance of yajna (Vedic Sacrifice) one needs an expert priest who is pure is mind, body and speech.  Having undergone disciplinary training and strict vows guided by the bonafide guru or acharya (realized saint who teaches by his own example) one becomes a vipra or an expert in not only the Vedic rituals and hymns, but is able to bestow blessings upon the participants of sacrifice by the grace of God.

Therefore we are not only concerned with the proper recitation of mantras and Vedic hymns and adherence to the rules of ritualistic ceremony, but we are keenly aware that unless the priest is himself pure, austere, devoted to God, self controlled and completely free of sin it is impossible to get tangible results from the performance of sacrifice such as Vivaha Yajna (the Hindu Marriage Rite).