1 Timothy 6:12:

Fight the good fight of faith, lay hold on eternal life, whereunto thou art also called, and hast professed a good profession before many witnesses.

See also:

You Have the Victory, Not the Advantage
Healing and Effort
How NOT to Deal with the Devil
What 1 John 1:9 Really Means
Must I Engage in Spiritual Warfare to See the Manifestation of My Healing?
Objection: Paul Told Timothy to Drink Wine Instead of Believing God for Healing