How To Lose the Assurance of Your Salvation by J.C. Ryle
Inconsistency of life is utterly destructive of peace of conscience. The two things are incompatible. They cannot and they will not go together.
If you will have your besetting sins and cannot make up your minds to give them up, if you will shrink from cutting off the right hand and plucking out the right eye when occasion requires it, I will engage you will have no assurance.
A vacillating walk, a backwardness to take a bold and decided line, a readiness to conform to the world, a hesitating witness for Christ, a lingering tone of religion, a clinching from a high standard of holiness and spiritual life, all these make up a sure receipt for bringing a blight upon the garden of your soul.