Charles Ellicott Commentary Matthew 5:48

"Ye therefore shall be perfect, as your heavenly Father is perfect." — Matthew 5:48 (ASV)

Be ye therefore perfect — Literally, You therefore shall be perfect — the ideal future that implies an imperative.

Your Father which is in heaven — A better reading gives, your heavenly Father. The idea of perfection implied in the word here is that of attaining the end or ideal completeness of our being. In us, that attainment implies growth, and the word is used (e.g., in 1 Corinthians 2:6; Hebrews 5:14) of men of full age as contrasted with infants. In God, perfection is not something attained but exists eternally; however, we draw near to it and become partakers of the divine nature when we love as He loves:

“Earthly power does then show most like God’s
When mercy seasons justice.”