"For by the grace given to me I say to everyone among you not to think of himself more highly than he ought to think, but to think with sober judgment, each according to the measure of faith that God as assigned."
In chapters 1-11 Paul gives in depth understanding to the gospel of grace. In verses 1-2, he outlined the practical personal implications for someone who follows Jesus. In these next verses, Paul moves toward the communal considerations of grace. How Christians should live together is what Paul is addressing in the following verses.
God has given each Christian a "measure of faith." Every follower of Jesus has some level of faith. Paul calls each Christian to assess himself realistically through the lenses of his faith.
In light of our faith in Christ, we should not think of ourselves more highly than we should. We should all remember our previous sinful state. There was nothing meritorious within ourselves that earned God's favor. We were all saved by grace through faith. We should all have "sober judgment" concerning ourselves.
Humility should be the characteristic that most describes us. We are nothing. Christ is everything. Life is all about glorifying him and him alone. Who are we that God should even think about us?
We should live humbly with one another. As we have received grace from Jesus, we should give grace to fellow followers of Christ.
That proves we understand the gospel of grace.