The pair clung to a large red cross for five hours before they were rescued.
Speaking to WIS, Clara Gantt said that she drove to attend a service at the Three Rivers Baptist Church in Irmo, around 6am.
In the heavy rain visibility was poor and the rains swept her car away according to Christian Today.
She told the news station: "I knew there was water out there, but I didn't know that it was going to stall me and it did.
Her car landed in a field next to the church where she tried to call the emergency services but could not get through.
She managed to speak to her family.
Clara Gantt said that she said to herself: "Dear Lord are you taking me home right now?" and she heard a voice inside her head that replied "No, not right now."
Travis Catchings came to rescue her, carrying rope and a harness.
He said: "I got [the door] open and got her out of the car.
"At that point we had to just sit there and wait."
His grandmother added: "There was a little church right there.
"Where my car stopped was right beside a huge red cross," Gantt said. "I was literally, after I got out of the car, holding onto the cross. I was clinging to the cross."
Gantt and her grandson both held on to a red cross until the rescuers arrived.
Prayers go out to everyone in Columbia, Charleston and other parts of South Carolina dealing with flood waters ?????? pic.twitter.com/a993JjE5OQ— Coach Jeff Scott (@coach_jeffscott) October 4, 2015
Mr Catchings did not think he would be rescued in time and so made a last phone call to his wife: "I really thought that was the last time that I was ever going to talk to her.
"I thought that was the last time that I'd ever going to hear her voice.
"It was the hardest phone call I've ever made. And I just told her that I loved her, and I didn't know what was going to happen."
"Jesus is my saviour.
"This story is not about me, it's about what He did to save me.
"And He set my feet on higher ground."