Calculate the number 866

Mr. Marlow was strolling through the country when he saw a stable with the most beautiful horse he ever laid eyes on. It was seventeen hands high and white, with rippling muscles and a fine, flowing mane. Mr. Marlow struck a deal to buy it from the owner who did, however, pass on one key piece of information.
"We are a religious family, Mr.Marlow, and we've instilled those values in our horse. To get him to gallop you must say 'Thanks God' to get him to stop you must say 'Our Father Who Art in Heaven," Settling into the saddle, Marlow said " Thanks God," and the animal took off. They rode for miles; suddenly they were coming up to a cliff. Unfortunately, Marlow couldn't remember the phrase to make the animal stop and tried every Biblical passage he could think of until, just a few feet from the edge of the cliff, he shouted, " Our Father Who Art in Heaven! The animal stopped instantly. Shaking and perspiring, Marlow reached into his pocket and pulled out a handkerchief. "Thanks God," he said as he mopped his brow...