Marriages usually take place in the bride's church so the Millars may, or may not, have been Presbyterian. Also, Presbyterians are free to choose their own congregation (and there are different forms of Presbyterianism so lots of churches) so they may not have attended the nearest Presbyterian church.
The marriage took place in Brigh Presbyterian Church but Samuel's residence at the time of the marriage is Cookstown (he may have been born elsewhere). The certificate says that Samuel's father was Thomas Millar, also a car driver.
How did you skip from Samuel back to his grandparents Thomas & Rose Ann Millar?