We have been cast down many times in this conflict, and still are; but we do not perish, for Christ has always triumphed and does triumph through us. Therefore, we firmly hope to obtain the victory against the devil through Jesus Christ.--Martin Luther, Galatians (Crossway, 1998), 120
This hope brings us sure consolation, so that in the midst of our temptations we take courage and say, 'Satan has tempted us before and by his deceit has provoked us to infidelity, contempt for God, despair, and so on; yet he has not prevailed, nor shall he prevail from now on (1 John 4:4). Christ is stronger and has overcome and continues to overcome that strong one in us, and he will overcome him forever.'
Still, the devil sometimes overcomes us in the flesh. But even then we may experience the power of a stronger person over that strong enemy and may say with Paul, 'When I am weak, then I am strong' (2 Cor 12:10).