38 Now to all Christians in general it has been said: "Be ye perfect as your Heavenly Father is perfect"; how much more then should the priest consider these words of the Divine Master as spoken to himself, called as he is by a special vocation to follow Christ more closely. Hence the Church publicly urges on all her clerics this most grave duty, placing it in the code of her laws: "Clerics must lead a life, both interior and exterior, more holy than the laity, and be an example to them by excelling in virtue and good works." And since the priest is an ambassador for Christ, he should so live as to be able with truth to make his own the words of the Apostle: "Be ye followers of me, as I also am of Christ"; he ought to live as another Christ who by the splendor of His virtue enlightened and still enlightens the world.
Source: Ad Catholici Sacerdotii (Vatican.va)