Athos proves that he is a loyal friend to d’Artagnan by getting arrested on his behalf. Evidently, the cardinal’s men have realized that d’Artagnan is involved in Madame Bonacieux’s plot, meaning that he is in danger—or, at least, he would have been if Athos didn’t go to jail in his place. Athos’s decision is a good example of the musketeers’ motto (“all for one, one for all”) in action.