This quotation is from Tennyson's famous poem "Idylls of the King" which deals with the life and adventures of King Arthur. These particular lines are from the section popularly known as 'The Passing of Arthur.'
"Bold Sir Bedivere," one of the most courageous 'Knights of the Round Table,' cries out in sorrow in the following words, just before the funeral barge in which the dying King Arthur has been placed is about to sail away forever:
For now I see the true old times are dead,"
He laments that the institution of 'The Knights of the Round Table' has completely disintegrated:
It is then that the dying King Arthur feebly but stoically remarks:
Lest one good custom should corrupt the world."
In history only one thing, namely, change is constant. King Arthur replies with calm fortitude to the inconsolable Sir Bedivere that however magnificent and worthy the institution of 'The Knights of the Round Table' might have been it is only appropriate in God's divine plan that it should pass away and give way to a newer institution because any institution which lives on and on will only become irrelevant and thus harmful to society.