St Jerome has preserved for us this anecdote from the 1st century:

"The blessed Evangelist John lived at Ephesus down to such an extreme old age that he was with difficulty supported in the arms of his disciples and so was carried to the church. And being unable to articulate many words, he was wont to utter each time to the congregation the simple words: 'Little children, love one another.'

At last his disciples and brethren were weary of hearing these words so often, and asked him: 'Master, wherefore ever sayest thou this an answer worthy of John: 'Because it is the commandment of the Lord, and if this only be done, it is enough' " (Commentary on Galatians, ch. VI).