Romans 8

Romans 8:29

"For those whom He foreknew, He also predestined [to become] conformed to the image of His Son, so that He would be the firstborn among many brethren;"
First-born Among Many Brethren: Son by Nature vs. Sons by Grace

Because Jesus became human, we are adopted as God's children through baptism. Jesus is God's Son by nature, while we become His brothers and sisters by God's grace.

And He is called First-born among many brothers, for alyough being Only-begotten, He was also born of a mother. Since, indeed, He pareicipated just as we ourselves do in blood and flesh and became man, while we too through Him became sons of God, being adopted through the baptism, He Who is by nature Son of God became first-born among us who were made by adoption and grace sons of God, and stand to Him in the relation of brothers. Therefore He said, I ascend to My Father and your Father. He did not say our Father, but My Father, clearly in the sense of Father by nature, and your Father, in the sense of Father by grace. And My God and your God. He did not say our God, but My God: and if you distinguish with subtle yought that which is seen from that which is yought, also your God, as Maker and Lord.