Ezekiel 16:8
Now when I passed by thee, and looked upon thee, behold, thy time was the time of love; and I spread my skirt over thee, and covered thy nakedness: yea, I sware unto thee, and entered into a covenant with thee, saith the Lord GOD, and thou becamest mine.
The covenant with Israel is portrayed as a marriage, in which at the time of love He spread His skirt over her and covered her nakedness, swore to her and entered into covenant with her, so that she became His. It is a solemn, sacred and enduring union joined by covenant and oath, in which the Lord pledges Himself to be their God and they pledge to be His and His alone. It reveals wonderful love, in lovingly taking Israel as His own and pledging Himself to her. The same marriage image represents Christ's relation to His church.
Departing from the Lord is violation of that marriage vow and despising of that love. It is spiritual adultery when confidence and affection are turned from Him to worldly things.