One of the great gospel truths is that we have been adopted as God’s children, his heirs together with Christ.

For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. And by him we cry, “Abba, Father.” The Spirit himself testifies with our spirit that we are God’s children.

- Romans 8:15-16

What does it mean to be a child of God?

Now if we are children, then we are heirs — heirs of God and co-heirs with Christ, if indeed we share in his sufferings in order that we may also share in his glory.

- Romans 8:17

J. I. Packer explains what it means to “share in [Christ’s] sufferings in order that we may also share in his glory”:

In this world, royal children have to undergo extra training and discipline, which other children escape, in order to fit them for their high destiny. It is the same with the children of the King of Kings. The clue to understanding all his dealings with them is to remember that throughout his life he is training them for what awaits them, and chiselling them into the image of Christ.

- J. I. Packer, Knowing God (Hodder & Stoughton, 2004) p. 251.

This chiselling may well be painful, but it is necessary for us if we are to attain to our “high destiny” as children of God.