Grown up, Mature, or Perfect

The Message paraphrase version NT
Mt 5:48  “In a word, what I’m saying is, Grow up. You’re kingdom subjects. Now live like it. Live out your God-created identity. Live generously and graciously toward others, the way God lives toward you.”

This passage is in context of loving our enemies, shown in the NKJV below.

Love Your Enemies

⁴³ “You have heard that it was said, ‘You shall love your neighbor^[g] and hate your enemy.’ ⁴⁴ But I say to you, love your enemies, bless those who curse you, do good to those who hate you, and pray for those who spitefully use you and persecute you,^{[h] 45} that you may be sons of your Father in heaven; for He makes His sun rise on the evil and on the good, and sends rain on the just and on the unjust. ⁴⁶ For if you love those who love you, what reward have you? Do not even the tax collectors do the same? ⁴⁷ And if you greet your brethren^[i] only, what do you do more than others? Do not even the tax collectors^[j] do so? ⁴⁸ Therefore you shall be perfect, just as your Father in heaven is perfect.

 In most translations, including the NKJV, you can see that the word translated in verse 48 is "perfect", as in "...you shall be perfect, just as your Father in heaven is perfect."

The Message version paraphrases the Koine Greek word "τέλειοι" as 'Grow up'. Its an interesting thing about the word. Because, depending on the context, it can be translated as "perfect", "complete", and/or "mature". 
Obviously, the translators of The Message took the meaning to be "mature", when they considered the context. 

However, they are not alone in translating the word differently. The Bible In Basic English as well as Weymouth New Testament versions, translate τέλειοι as "complete". 

If you think about how the Greek language applied the term, it really makes more sense to think that Jesus is saying that His disciples will be mature or complete, in the way we are to love one another. We are to be just like God the Father. He is mature and complete in His expression of love to mankind, who have rebelled and made ourselves enemies of the Him.