Why do bishops place a cross before their names?

You may have noticed that bishops sometimes place a small cross before their names (e.g. + John Gilroy or +JHG).

Why is this?

The custom derives from the era when medieval bishops would humbly sign their names by prefixing the title “Sinner” or “Sinful Servant”. So Anselm of Canterbury might sign his letters in this way:

Your Sinful Servant,
Anselm

The cross prefix is merely a symbol of one being a “sinner” – that is one in need of the redeeming cross of our Lord Jesus Christ.