John Chrysostom

"For men verily swear by the greater, and an Oath for confirmation is to them an end of all strife. But God because He could swear by no greater, sware by Himself." Well, who then is He that sware unto Abraham? Is it not the Son? No, one says. Certainly indeed it was He: however, I shall not dispute thereon. So when He the Son sweareth the same oath, "Verily, verily, I say unto you," is it not plain that it was because He could not swear by any greater? For as the Father sware, so also the Son sweareth by Himself, saying, "Verily, verily, I say unto you." He here reminds them also of the oaths of Christ, which He was constantly uttering. "Verily, verily, I say unto thee, he that believeth on Me shall never die."

What is, "And an oath for confirmation is to them an end of all strife"? It is instead of, "by this every doubtful question is solved": not this, or this, but every one.