Under South Carolina law, it is not illegal for a person under the age of 21 to consume alcohol for religious purposes.  S.C. Code Section 20-7-320.  However, the alcohol must have been purchased legally. 

As to whether someone under the age of 21 would then get into trouble for driving home, that would depend on how much alcohol the person consumed.  Ordinarily, religious ceremonies that involve alcohol use only small amounts (like a small sip of wine for Communion) that should not interfere with a person's ability to drive.  However, if a person under the age of 21 is caught driving with a blood-alcohol level of .02% or more, that person's driver's license will be suspended.  S.C. Code Section 56-1-286.