Belonging to God

Here is a quote from Charles Ryrie in his book, Basic Theology, on Satan's influence.  (Linked to his book for sale on Amazon).  I unfortunately have not read the book but came across this quote the other day.  How encouraging is this?

Whatever Satan or demons can do to a believer, and whether from without or within as their base of operation, their control cannot be permanent and eternal.  John clearly declares that the evil one cannot "touch" the one who is born of God (1 John 5:18).  The word "touch" here involves the purpose of harming -- Satan cannot harm the believer...Satan can never hang on to the believer with the purpose of harming him, for that believer belongs eternally and irrevocably to God. (pg. 168)

Satan cannot harm us (spiritually) because we belong "eternally and irrevocably to God."  I find that truly heartening.