If you want to be more polite you could use His Reverence.If the letter is formal, the salutation would be The Reverend Father Walter; for a personal letter, the salutation would be Father Walters, or if you know him better, Dear Father Walters .If you are writing to priest who is a member of a religious order, you would add the initials of his community after his name, e.g. The Reverend Philip Am