15:39 And it shall be to you for a fringe, that ye may look upon it, and remember all the commandments of the LORD, and do them; and that ye seek not after your own mind and your own ability to observe, with which you used to commit fornication:  
 

Whoring refers to idolatry.  With our very limited ability to observe and our tainted ability to reason, why do we challenge God.  Yet, it happens.  This is probably the most common of all forms of idolatry.