"If you accept that number," he said, "you sell your soul to the devil."
Bob LaCourciere, vice president of sales and marketing for Revstone Corp., which owns Contech Casting, said that Slonopas' W-2 was labeled with 666 by the company that handles Contech's payroll. It refers, he said, to the order in which the forms were mailed out.
This isn't the first time that the satanic number has caused Slonopas trouble at work.
During his first day on the job in April 2011, Slonopas was supposed to be assigned the number 668 to use when he clocked in. But the human resources department gave him the wrong number 666 instead.
Slonopas, who said he became a born-again Christian about 10 years ago, complained and was given a new number.
In July 2011, the company changed time-clock systems, and once again Slonopas got 666. This time he quit. The company apologized and he returned to work a few days later.
This latest incident with the W-2 baffled company spokesman LaCourciere. He could not believe it had happened again.
"I am completely at a loss for words," he said.
The number 666 first appears in Chapter 13 of the New Testament Book of Revelation, which describes a satanic figure called the beast or Antichrist who takes over the world and stamps everyone with a mark bearing the number 666. According to Revelation, no one will be able to buy or sell anything without that number stamped on them.
That's caused people to fear anytime that number pops up, said Jay Phelan, senior professor of theological studies at North Park University in Chicago. "It's seen as a very dangerous number."
For believers such as Slonopas, who take the Book of Revelation literally, any tie to 666 is a betrayal of their faith. Phelan said he understands why Slonopas quit.
"It's a desire to be loyal to his faith and to not be identified with the Antichrist," he said. "The company ought to find a way to cut him some slack."
Amy-Jill Levine, professor of New Testament and Jewish studies at Vanderbilt University Divinity School, said the writer of Revelation was using a technique called "gematria" in which letters have numerical values to refer to a Roman emperor as the beast.
She said that during the past 2,000 years, readers of Revelation have tried to use 666 to figure out who the Antichrist is. Among the candidates were political figures such as Hitler, Ronald Reagan and Barack Obama and corporations such as Procter & Gamble and IBM.
The number has caused problems for at least one other worker in the past. In 2011, a factory worker from Georgia named Billy Hyatt sued his former employer after he was fired for refusing to wear a sticker with 666 on it. The sticker referred to the number of accident-free days he'd had on the job.
Slonopas, though, said he has no interest in suing anyone. All he wants is for his former employer to give him a new W-2 without a satanic number on it. Otherwise, he said, he can't file his taxes.
He shakes his head when asked if he'd go back to work for Contech, even if the company gives him a new W-2. That would send the message that he sold out his faith for money.
"God is worth more than money," he said.
LaCourciere said the firm planned to mail out a new W-2, in a plain envelope, by the end of the day on Tuesday. The company also wants to rehire him.
"We'd love to have him back," he said.