Albert Barnes Commentary Matthew 5:45

"that ye may be sons of your Father who is in heaven: for he maketh his sun to rise on the evil and the good, and sendeth rain on the just and the unjust." — Matthew 5:45 (ASV)

That you may be the children of your Father. In Greek, the sons of your Father. The word son has a variety of meanings (see Barnes on Matthew 1:1).

Christians were called the sons or children of God in several of these senses: as his offspring, as adopted, as his disciples, and as imitators of him. In this passage, the word is used because, in doing good to enemies, they resemble God. He makes his sun to rise on the evil and the good, and sends rain, without distinction, on the just and the unjust. So his people should show that they imitate or resemble him, or possess his spirit by doing good in a similar way.