Being just simply means being with Christ and in Christ. And this suffices. Further observances are no longer necessary. For this reason Luther's phrase: "faith alone" is true, if it is not opposed to faith in charity, in love. Faith is looking at Christ, entrusting oneself to Christ, being united to Christ, conformed to Christ, to his life. And the form, the life of Christ, is love; hence to believe is to conform to Christ and to enter into his love. So it is that in the Letter to the Galatians in which he primarily developed his teaching on justification St Paul speaks of faith that works through love (cf. Gal 5: 14).
General Audience, 19 November 2008
ADDED LATER: I'm reminded, actually, of Jonathan Edwards:
And besides, as the word condition is very often understood in the common use of language, faith is not the only thing in us that is the condition of justification. For by the word condition, as it is very often (and perhaps most commonly) used, we mean anything that may have the place of a condition in a conditional proposition, and as such is truly connected with the consequent, especially if the proposition holds both in the affirmative and negative, as the condition is either affirmed or denied. If it be that with which, or which being supposed, a thing shall be, and without which, or it being denied, a thing shall not be, we in such a case call it a condition of that thing. But in this sense faith is not the only condition of salvation and justification. For there are many things that accompany and flow from faith, with which justification shall be, and without which, it will not be, and therefore are found to be put in Scripture in conditional propositions with justification and salvation, in multitudes of places. Such are love to God, and love to our brethren, forgiving men their trespasses, and many other good qualifications and acts. And there are many other things besides faith, which are directly proposed to us, to be pursued or performed by us, in order to eternal life, which if they are done, or obtained, we shall have eternal life, and if not done, or not obtained, we shall surely perish.
Edwards goes on to say, of course, that faith is the only condition of justification in being the sole cause (in us) of justification, the thing that renders it fitting for us to have it; but things other than faith are conditions of justification in the more ordinary sense of inseparably attending justification.