You therefore must be perfect, as your Heavenly Father is perfect (ESV)
Ἔσεσθε οὖν ὑμεῖς τέλειοι ὡς ὁ πατὴρ ὑμῶν ὁ οὐράνιος τέλειός ἐστιν. -- Matt. 5:48
Today in church we read selected verses from the Sermon on the Mount. As we were doing so, I found myself puzzled. As noted above, the ESV translates Matt. 5:48 as an imperative, as does the NIV. However, the verb (Ἔσεσθε) is not an imperative, but a future. So, any translation that makes the verb imperative misses the mark grammatically. It should be translated as "You will be perfect...," with a textual note that it is future, as the English "will be" can be understood as a command (oh, English...). Preferably, the translation should read "You shall become..."
Translating as an imperative, though, has greater than grammatical problems. It changes the tenor of the passage entirely. In the ESV and NIV, the discussion of loving one's enemies becomes an impossible standard, for who can attain to the measure of the Father? Since a common Protestant understanding of what "works" do in the human life is to underscore our will's inability, and therefore the impossibility of attaining to the divine standard, it is understandable why the passage would be mistranslated such. However, if this is the reason behind using an imperatival form, it is the tail (Protestant theology) wagging the dog (the translation of the text). Since we are to read Scripture through the Apostolic Deposit (regula fidei), this isn't necessarily a bad thing. However, in this case, it verges on it.
Read with a proper translation, the passage has a beautiful promise contained within it. If we love our enemies, we may (subjunctive) be sons of our Father in heaven (v. 45). To be a son is to be like the father. So, by loving our enemies, we open up the possibility (subjunctive) of being divine sons. However, our Lord is not content to leave us with the possibility. Instead, if we love our enemies, we will become (future) complete, mature, perfect, as the Father already is. Not only is the possibility opened up to us, but, as we practice love of enemies, we are transformed more and more into the Image of the Father. This promise of God-likeness (1 John 3:2-3, etc.) comes from the Lord Himself, so it is assured. In other words, love your enemies for in doing so you show that you are sons of the heavenly Father and are participating in His perfection, bit by bit, little by little, as the Spirit empowers us so to do.
If we want to remember the effects of sin on our lives and the difficulty of attaining to God's standards, it is better to use passages such as John 15:5: "without Me [Jesus] you can do nothing."