This verse is an instruction to shift the focus or the standard of measurement away from what we are accustomed to using for our lives and base it on a whole new paradigm. Instead of measuring “success” by how much we can achieve or by external results, Paul is saying that in the life where the minds are renewed and the lives are being transformed that the true standard of quality is found internally. Furthermore, the measurement is to be done by the person themselves, not judging others around them.
The world's system uses comparisons between people to establish credibility and honor. It is the system of artificial value and external emphasis. It is based on the false trinity of gradation through the lenses of Economics, Kingship and arbitrary Law (what we term civilization). In the body of Christ that Paul is unveiling here there is to be no contamination by the principles used in the world. No one is to think of himself as better or more valuable than anyone else but neither are they to think of themselves as less either. Under the system of God's “kingdom” they are learning to live based on a healthy family model, not the artificial differentiations of the world.
In the world the motivation for life is a desire for control and domination over others. Power is the object of pursuit in the desperate struggle to feel valued and satisfied. But in God's family this is not to be found in the heart or in relationships. Every single person has been given by God a measure of faith to start with and their life and the exercise of their gifts is to be measured by their faithfulness in keeping within that context. Just a little farther on in Romans Paul states a very important principle: whatever is not from faith is sin. (Romans 14:23)
Since we have gifts that differ according to the grace given to us, each of us is to exercise them accordingly: ...according to the proportion of his faith. (Romans 12:6)
In the following verses that outline various gifts that God empowers the members of the body to carry out in loving service for the rest of the body, each one is to be seen in this context as the measurement of authenticity. In the body of Christ, the family of God, we are not to use the world's system of external measurements to determine our usefulness or value but we are to live within our means of faith.
I find it very relevant that if I try to exercise whatever gifts I have independent of or beyond the faith I have received then I have moved beyond authenticity and am now into the area of sin. I am all too aware of many times when I have done just that. Whenever I am writing things that I am learning from the Spirit or sharing things in conversation with others, there are times when I suddenly sense that I am moving beyond the reach of my “proportion of faith” that is in my heart and then my head is confidently lurching to take the lead without listening to the reservations of my heart or paying attention to the quiet voice of the Spirit warning me to cease and desist. Whenever I allow that to progress I always end up regretting that decision and realize that I have once again caused damage to part of the Body.
It seems to me as I meditate on these verses that they are reinforcing the principle that I have been learning that is so important and vital in living the Christian life, the principle of living from the heart that Jesus gave me instead of by my own smarts and abilities.
Trust in the Lord with all your heart and do not lean on your own understanding. In all your ways acknowledge Him, and He will make your paths straight. (Proverbs 3:5-6)