How do we word the invitations when the ceremony is small and not all guests are invited?

One option is to have two different texts printed (one with an invitation to the reception only, and a different one with an invitation to both the ceremony and the reception). Another solution would be to print only one text, inviting everyone to the "wedding reception of" with "request the pleasure of your company" instead of "request the honour of your presence". A separate card would be added to the invitations of those guests you would like to attend the ceremony, with wording such as:

Mr. and Mrs. John David Taylor
request the honor of your presence
at the marriage ceremony
at six o'clock
St. Paul's Church