From his lectures on Romans (circa 1515): ‘God does not want to redeem us through our own, but through external, righteousness and wisdom; not through one that comes from us and grows in us, but through one that comes to us from the outside; not through one that originates here on earth, but through one that comes from heaven. Therefore, we must be taught a righteousness that comes completely from the outside and is foreign. And therefore our own righteousness that is born in us must first be plucked up.’ LW 25:136
Luther means here that we need the righteousness of Christ – that righteousness by which we are justified because our own righteousness must be plucked up for the filth that it is. Our own righteousness can be compared to a blind man who is holding in his hand what he believes to be the consumable cure for his sin. He just does not realise he is holding his own dung in his hand. This is why we need an external or alien righteousness, one that comes as a gift – by faith alone – from Jesus Christ.