Adultery refers to sexual union between a married person and someone other than his or her spouse. The penalty for adultery was severe (see Deut. 22:22, Lev. 20:10).
From the verse, "The … adulterer waits for twilight saying, No eye shall see me" (Job 24:15), the Talmud identifies the adulterer as a practical atheist, since he does not say, "No man shall see me, but no eye - neither the eye of one below nor the eye of Him above."
Yeshua our Messiah identified the root condition of adultery as a problem with the heart: "For out of the heart proceed…adulteries." The heart's true affections are evidenced by the use of one's eyes (see Matt. 5:27-28).
Adultery is a grave sacrilege, since it not only violates the sworn promise of parties to a sacred covenant, but perverts the picture of our union with God Himself. As Paul wrote to the Kehillah (assembly or "church") at Ephesus, "We are members of his body, of his flesh, and of his bones. For this cause shall a man leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh. This is a great mystery: but I speak concerning Messiah and the church" (Eph. 5:30-32).