Jesus is " the author of eternal salvation [' salvation for the age'] unto all them that obey him"
Matt.7:24-29;John.15:1-15]. A Christian must follow that which the Lord has commanded [which will result in his keeping himself unspotted by the world].
Sin is disobedience to that which the Lord has commanded. And though Christians- presently in a body of flesh, housing the old sin nature-may fall, cleansing is available. That's why Christ is presently exercising the office of High Priest in the heavenly sanctuary [cf.John 13:8-10; Heb. 10:19-22; 1 John 1:6-2:2].
Sin must be dealt with prior to the battle. Then, believing that the Lord will do exactly what He has promised,victory after victory can ensue as the person moves forward, keeping his eyes fixed on the goal. There can be no such thing as defeat if one moves in accord with the Lords instructions.