"Do ye think that the scripture saith in vain, The spirit that dwelleth in us lusteth to envy?"
A better description could be that Paul asks a rhetorical question that define opposites.
In our lingo, we would probably hear it this way...'Or do you think that the Scripture has vainly said....'
Reading the previous verse gives better understanding to verse 5.
"Ye adulterers and adulteresses, know ye not that the friendship of the world is enmity with God? whosoever therefore will be a friend of the world is the enemy of God."
Then we can understand much easier by reading thins in context, knowing that Paul is exposing the worldly pagan practices of the fertility goddess "Astaroth."
"You adulteresses, do you not know that friendship with the world is hostility toward God? Therefore whoever wishes to be a friend of the world makes himself an enemy of God. Or do you think that the Scripture speaks to no purpose: “He jealously desires the Spirit which He has made to dwell in us”? (NASB)
Astaroth (also Ashtaroth, Astarot and Asteroth), in demonology, is the Great Duke of Hell, in the first hierarchy with Beelzebub and Lucifer; he is part of the evil trinity. He is a male figure most likely named after the Mesopotamian goddess Ishtar.