In Matthew 5:48 Jesus says, "You are to be perfect as your heavenly Father is perfect."
Today we can understand Matthew 5:48 at least these three ways:
Perfection is Not by Performance
Perfection does not come by behavior. When we assume that our behavior is the manner in which we attain any spiritual thing we miss what Christ has done for us.
Jesus taught the law before His death to reveal the desperate need for Himself and His coming sacrifice so people would be ready to trust His sacrifice. It is impossible for us to keep God's old covenant laws. Jesus said "be perfect" to make that evident to the people of His time who felt they could keep the law fully. "Be perfect as God is perfect" is the ultimate "I can't do that." Jesus was making it evident to people's hearts that righteousness could not be attained by adherence to the law (behavior). Instead of outright saying this Jesus wisely allowed the people time to discern this in their own hearts.
We are Perfect in Our Process
We don't expect children to be as mature as adults. In keeping with your development in Christ God can look at you as perfect in the place you are in your life. God isn't stupid. He knows you fully and He doesn't expect you to be able to do or be more than you are today. God is satisfied with you.
We are Perfected by The Prince of Peace
Only a child of the Father can be perfect like the Father. When you are born again, by trusting Jesus, you are perfected in Christ. "By one offering He has perfected for all time those who are made holy" (Hebrews 10:14).
All of God's children are "made holy." They are all called "saints" which means "holy ones." When we first trust Jesus we are perfected for all time, thus He makes us perfect as the Father is perfect. So this perfection is not something we strive to attain but rather what Christ has already attained for us & gives to us at salvation.
We can actually see that, as His child, it is impossible to not be perfect in God's eyes! And that from two different considerations of what "perfect" can be. When we trust Jesus for our perfection rather than our performance we experience the abundant life Jesus talked about. The Christian never has to try to become. The Christian simply trusts that they already are because of what Jesus Christ has done.