"There is one thing that all Satan's cunning and all the snares of temptation cannot take by surprise - an undivided will. What Satan spies with keenness of sight as his prey, what all temptation aims at certain of its prey, is the ambiguous. When unclarity resides, there is temptation, and there it proves too easily the stronger. Wherever there is ambiguity, wherever there is wavering, there is disobedience down at the bottom . . . We must either love or hate, and not to love is to hate. So hostile are these two powers that the slightest inclination towards the one side becomes absolute opposition to the other."
Soren Kierkegaard in Provocations: Spiritual Writings of Kierkegaard, p. 11-12.
This reminds me of both Jesus' and James's teaching. See Matthew 6:24 and James 4:4-5.