‘Jesus said in the sermon on the mount, “Be ye therefore perfect even as your Father in heaven is perfect” (Matthew 5:48).
Does God command the impossible? If He did, that would make Him a despot, a tyrant. But God is the Benevolent Moral Governor of the Universe. When He gives us a command, He provides a promise or the means to enable us to fulfill the command. Hebrews 10:14 give us the means to perfection, “For by one offering (blood sacrifice) He hath perfected forever them that are sanctified.” When one repents (forsakes sin) and puts his faith in the blood of Jesus Christ that cleanses from all sin, he is morally perfect.
Understand, our Lord commanded a moral ethical perfection, not an absolute perfection. Only God is absolutely perfect. We are not perfect in knowledge and understanding. Therefore, we are susceptible to making mistakes. However, a mistake is not a sin. Sin is intentional transgression of the law; a mistake is an unintentional deviation from the truth or right conduct. If I intentionally misquoted a scripture to win an argument, it would be sinful. If I did it unintentionally because of my imperfect memory, it would be a mistake.
A Christian may have faults. For example, he may worry; but only if his worries or fears prevented him from doing the will of God, has he sinned.
Moral perfection is a purity of motives and intentions. “Blessed are the pure in heart for they shall see God” (Matthew 5:8).
Christian perfection is loving God with all of your heart, mind, soul and strength and loving your neighbor as yourself. On these two commandments hang all the law and the prophets. This is all that God requires of us.’
source: Jed Smock, Who will Rise Up? (1985), p. 221-222.