This is with regards to the physical body, not the spiritual adoption as sons.
You are confusing things
To redeem those who were under the law, so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of his Son into our hearts, crying, “Abba! Father!” So you are no longer a slave, but a son, and if a son, then an heir through God.
Galatians 4:5-7
Right, we can be called sons now because of the certainty of the future adoption of sons. It is that certain! Praise the Lord! The Holy Spirit is the seal that guarantees us the future adoption. This is the context of predestination. Our destination is to be conformed to the image of Jesus. That’s the future adoption. Check this out...
1 John 3
1 Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.