Albert Barnes on Matthew 5:45

That ye may be the children of your Father. In Greek, the sons of your Father. The word son has a variety of significations. Mt 1:1. Christians were called the sons or children of God in several of these senses: as his offspring; as adopted; as his disciples; 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 good, and sends rain, without distinction, on the just and unjust. So his people should show that they imitate or resemble him, or possess his spirit by doing good in a similar way.

(l) "sun to rise" Job 25:3