Formally it would be "The Reverend John Smith" - but only in the third person. Note that the definite article is required, as are both first name and surname. If you are using the second person, for example when addressing him in speech or writing, the correct form of address is "Mr Smith".
Either may be appropriate. A Catholic Answer: Catholics customarily address Catholic priests as Father prefixed to his surname, or more familiarly Father prefixed to his given name. (Father Murray; Father Schaeffer; or Father Bob; Father Ted).
What is the proper etiquette to addressing a Reverend in a letter?
Rev. when in written form. Reverend in spoken form.
Reverend & Mrs.
Dr. and Mrs.............
'Reverend' is used as a title or form of address to members of the clergy.
You could address it to Reverend and Reverend Smith. The alternative would be the Reverends Smith.
If you do not know the appropriate mode of address for his or her denomination (pastor, reverend, brother, father, etc.) then "Doctor" would be appropriate. Usually "Reverend Doctor" is the formal address, as it is less misleading regarding profession.
Addressing a husband and wife who are both Reverends: Reverends John & Jane Doe.
The proper way to address a label for a retired priest would be "Reverend [First Name] [Last Name]." The title "Reverend" is appropriate for a retired priest as a sign of respect for their past role in the clergy.
When emailing a Reverend: Reverend John Doe.
Doctor.
The proper address of an abbot/abbess is: "The Right Reverend Abbot/Abbess [(canonical) First Name; Family Name is optional]"