Wow! What a command! Be sure to read the scriptures preceding this one so you can read in context.
Life Application Study Bible, NIV, footnotes:
How can we be perfect?
1. In character. In this life we cannot be flawless, but we can aspire to be as much like Christ as possible.
2. In holiness,. Like the Pharisees, we are to separate ourselves from the world's sinful values. But unlike the Pharisees, we are to be devoted to God's desires rather than our own, and carry his love and mercy into the world.
3. In maturity. We can't achieve Christlike character and holy living all at once but we must grow toward maturity and wholeness. Just as we expect different behavior from a baby, a child, a teenager, and an adult, so God expects different behavior form us, depending on our stage of spiritual development.
4. In love. We can seek to love others as completely as God loves us.
We can be perfect if our behavior is appropriate for our maturity level -- perfect, yet with much room to grow. Our tendency to sin must never deter us from striving to be more like Christ. Christ calls all of his disciples to excel, to rise above mediocrity, and to mature in every area, becoming like him. Those who strive to become perfect will one day be perfect, even as Christ is perfect (1 John 3:2,3).