To realise God is to be devoted to Him. The magnitude of His work, His greatness, His compassion and many such extremely heart-drenching qualities come into realization, due to which one feels more and more devoted to God.
Such pure love and the firm belief of a devotee towards God puts him in such unblemished solid state of right knowledge. His conduct therefore is flawless (as it is driven by such knowledge).
Note: It is not the greatness of the devotee, but it is the natural outcome of being associated with God. Just like association with a good friend has a good impact on one’s conduct, deep association with God moulds one to an exemplary state.