Lee Irons has responded to a post by Mark Jones:
Most of this doesn't interest me. I'm just going to comment on two or three of his statements:
For if we are accounted and accepted as righteous for Christ’s sake alone, then we are righteous, and being righteous means we are legally entitled to the reward of righteousness, namely, eternal life. To say that we need to add other conditions or qualifications would be to deny the sufficiency of Christ’s righteousness. It is to imply that Christ’s righteousness is not sufficient to qualify us to attain heaven. I am confident that Piper would disavow that implication with vehemence.
i) "Qualify" is Lee's word, not Piper's.
There's more to salvation than the forensic dimension. Salvation is not reducible to justification. Salvation is not reducible to the Cross.
ii) Traditionally, the sufficiency of Christ's righteousness stands in contrast to the Roman system of human merit. And there's no doubt that Christ's righteousness is sufficient.
But in Reformed theology, salvation is tightly Trinitarian. The work of Christ is not self-sufficient in isolation to the Father's work and the Spirit's work. These are integrated. The work of Christ is not independently sufficient. Rather, the work of the Father, Son, and Spirit in our salvation is interdependent. Reformed soteriology is Christocentric, not Christomonistic.
One wonders if this doesn't reflect "the Escondido Theology", with its quasi-Lutheran orientation. It's striking that D. G. Hart sides with Irons rather than Jones in this dispute. Moreover, Lutheran apologist Jordan Cooper wrote a supportive post, which Lee said was "excellent."
All of that is to say, the best of the Reformed tradition generally thinks it is better and safer to define faith as the instrument of justification rather than as the condition of justification.
But I would urge people, if they use it, to immediately clarify the sense in which they are using it. Preferably, we should not use it at all. It’s too ambiguous, as Owen said. We should use instrument instead—just as the Westminster Confession does. Besides, if faith is an instrument, then it is in some sense a condition. But not every condition is a mere instrument. So “instrument” is better because it is more precise.
Sure, you can define faith as an "instrument." But if you do that you, then have to define "instrument." What do most people think when they hear the word "instrument"? An electric guitar? Both "condition" and "instrument" need to be defined.
Moreover, I don't think "instrument" is "better and safer" than stating that faith is a necessary condition of justification.