Matthew 5:48 says "You therefore must be perfect, as your heavenly Father is perfect" (ESV). The word perfect is a word that can be very confusing to read because the Lord knows that we, as humans, are not perfect; hence, He sent His son to die for our sins. If God is aware of our imperfection, why would the Bible instruct us to be something unattainable?
First, we need to discuss what the word 'perfect' means. According to Webster's dictionary, perfect means: 'being entirely without fault or defect', and 'satisfying all requirements'.
Starting with the first definition, 'being entirely without fault or defect', it would appear as though the Bible is saying to be without flaw, and 'flaw' would be synonymous to 'sin': 'being entirely without sin'. As any Christian would know, this would be God's hope for his children: that we would all be without sin. The problem is that we know that it's impossible to be without sin, going all the way back to the beginning with Adam and Eve. The world was without sin, or 'perfect', until Adam and Eve ate the forbidden fruit.
How can God expect us to be without sin, when being all-knowing, he is already aware that people are not without sin? Is God being unreasonable? I don't think so considering that John 3:17 says, 'for God did not send his Son into the world to condemn the world, but in order that the world might be saved through him' (ESV). God sounds extremely merciful and reasonable, and just, to me. So, if he is merciful, and reasonable, and just, how can he ask us to be what we can never be?
The second definition says, "satisfying all requirements". According to the Ten Commandments, our requirements include; "You shall have no other gods before me', 'you shall not make for yourself a carved image [...] of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth", "you shall not bow down to them or serve them", "you shall not take the name of the Lord your God in vain", "remember the Sabbath day, to keep it holy", "honor your father and your mother", "you shall not commit murder, you shall not commit adultery, you shall not steal, you shall not bear false witness against your neighbor, you shall not covet your neighbor's house or your neighbor's wife, or anything that is your neighbor's" Exodus 20:3-17 (ESV).
The goal is definitely to obey all of the commandments. God wouldn't have commanded these things if he didn't want us to follow them. But, does following the commandments make us perfect? Not necessarily. 1 John 1:8-9 says, "If we say we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from the unrighteousness" (ESV).
What is the key then to being perfect? How can we, as sinful humans, be perfect as the Lord was perfect? The key is one word. Jesus.
God knows that we sin. God knows that we are not perfect. God knows that there is no way for us to be perfect without being saved from our sins. That's what Jesus did for us. He was crucified on the cross so that our sins could be cleansed. Jesus defends us to the Father so that we can be seen as worthy of someday dwelling in Heaven with Him.
1 John 2:1-5 says "My little children, I am writing these things to you so that you may not sin. But if anyone does sin, we have an advocate with the Father, Jesus Christ the righteous.He is the propitiation for our sins, and not for ours only but also for the sins of the whole world [...] whoever says “I know him” but does not keep his commandments is a liar, and the truth is not in him,but whoever keeps his word, in him truly the love of God is perfected" (ESV).
The love of God is perfected. Through Jesus we are perfected. We can't do anything without Jesus. No matter how hard we work to be perfect, and not sin, and try to be holy, we can't achieve this without Jesus Christ.
The Lord is not being unreasonable when he expects us to be perfect, like himself. He gave us the means to be perfect. We just need to accept the gift that God has given us.
"For all have sinned and fall short of the glory of God, and are justified by his grace as a gift, through the redemption that is in Christ Jesus" Romans 3:24.