To "fulfill" something means to realize it's original intent and potential, while to "obey all commandments perfectly" means to never sin while doing all you are required. To drive home the point I'm making - that there is a real conceptual distinction between the two - consider a few examples:
- When we say a prophecy was "fulfilled," that means it came to pass, yet this obviously has no logical connection to perfect obedience. Prophecies have been fulfilled about and by evil men, which is impossible if it meant they were perfectly obedient (e.g. see Acts 1:15ff of how Judas' sins fulfilled the Scriptures!).
- A prophecy or pre-figurement not about you cannot be fulfilled by you, nor does it depend on your obedience. For example, Paul says Christ is our Passover Lamb (1 Cor 5:7), something that never applied to men as individuals nor as Jews observing Passover. Another good example is when Jesus says, "Everything must be fulfilled that is written about me in the Law of Moses, the Prophets and the Psalms. ...This is what is written: The Messiah will suffer and rise from the dead on the third day..." (Luke 24:44ff). Clearly, Jesus fulfilled the Law, but not in a way we were ever intended to. Jesus is the one and only Messiah, we are not called to be a Messiah.
- A man becomes a Jew by circumcision, yet Jesus didn't get circumcised in place of saved Jews (who were already circumcised). Similarly, the Law forbids eating certain foods, such as pork, yet Jesus didn't abstain from pork in place of Gentile Christians (who never had to abstain from pork, and in fact many Christians eat pork, and this has no bearing on sin or salvation whatsoever). Such examples could be multiplied (e.g. Jesus getting Baptized and Christians getting Baptized). Thus, it cannot be said Jesus "fulfilled the Law in our place" is synonymous with saying Jesus "kept all the commandments in our place," since the latter is manifestly false.
- Saint Paul was very clear that Christians are called to fulfill the Law (Rom 13:8ff, Gal 5:14): "Let no debt remain outstanding, except the continuing debt to love one another, for whoever loves others has fulfilled the law. The commandments, 'You shall not commit adultery,' 'You shall not murder,' 'You shall not steal,' 'You shall not covet,' and whatever other command there may be, are summed up in this one command: 'Love your neighbor as yourself.' Love does no harm to a neighbor. Therefore love is the fulfillment of the law." If Jesus has to fulfill the Law "in our place" because we cannot, then Paul is either deluded or lying about what he's calling believers to do. Saint Paul's point here is neither about never sinning nor obeying all the commandments perfectly, but rather pointing out that Love is what all those individual commands was pointing to as the full realization of what God was trying to get across. Christians, who were sinners prior to their conversion, and at times still struggle with sin, are called to (and must) fulfill the Law as Paul describes above.
- As with the previous point, Christians are called to fulfill the Law in a way not humanly possible, since it rises above our natural human abilities (even Adam couldn't do them). One prime example of this is found in Romans 2:28f, "A person is not a Jew who is one only outwardly, nor is circumcision merely outward and physical. No, a person is a Jew who is one inwardly; and circumcision is circumcision of the heart, by the Spirit, not by the written code. Such a person’s praise is not from other people, but from God." What pleases God is a person who is inwardly transformed by the Power of the Indwelling of the Holy Spirit, something man lacks the ability to do, and this is what makes man a real Jew and truly circumcised (the fulfillment of each). Saint Paul is saying circumcision is required for salvation, just not the one made by human hands (cf. Col 2:11f). This isn't about us keeping all the commandments perfectly, nor is this about Jesus doing this in our place.
- The Bible never says anything to the effect of Jesus "kept the commandments in our place." True, Jesus never sinned and did keep all the commandments, and even fulfilled the Law in a unique sense, but nowhere does Scripture ever say this was done "in our place."
While many don't realize it, the doctrine of Justification by Faith Alone as the Reformers taught it hangs in the balance on many factors, including this one, so it's critical to know about and refute. Anyone who researches Protestant arguments on this notion of Christ's "Active Obedience" will see this error being frequently employed when they appeal to various Scriptural texts. Those Catholics who can understand this crucial concept will be on their way to properly explaining the true Biblical position (and refuting any Protestant errors) regarding the doctrine of Justification.