"48Be ye therefore perfect, even as your Father which is in heaven is perfect." (Matthew 5: 48)
"Be perfect, even as Your Father in heaven."
Meditate on this verse from the Sermon on the Mount one more time.
"Be perfect, complete, without defect."
We human beings are fallen because of Adam. Where does anyone get the idea that we can be perfect, let alone make ourselves perfect, through our own efforts?
We cannot.
Once again, Jesus was bringing the Old Covenant law back to its pristine and humanly impossible fullness, so that Israelites whom He was witnessing to would understand that they cannot earn or attain God's righteousness, and moreover they would despair of viewing Jesus as an example to follow and thus receive Him as a Savior to receive from.
Consider that when Jesus died on the Cross, He declared the following glad tidings:
"[B]ut go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God." (John 20: 17)
Notice that Jesus did not say "My disciples", but "my brethren."
He intimated very clearly that they were now adopted children with Him.
When Jesus saw them, He gave them the Holy Spirit:
"22And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost." (John 20: 22)
Just as God breathed into Adam to make him a living soul, so Jesus breathed in His disciples, now made brethren, to make them one with Him.
Paul writes at length what Jesus did for us at the Cross:
"For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him." (2 Corinthians 5: 21)
We are now the righteousness of God.
"For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ." (Romans 5: 17)
Read on these wonderful passages:
"To the praise of the glory of his grace, wherein he hath made us accepted in the beloved." (Ephesians 1: 6)
And if these verses are not clear enough, consider this beautiful promise:
"For by one offering he hath perfected for ever them that are sanctified." (Hebrews 10: 14)
We are now perfected in Christ, and we are now perfect, as our Father in Heaven, because:
"As He [Christ] is, so are we in this world!" (1 John 4: 17)