1) "Man adopts to supply a defect, because he has no children of his own, but God does not adopt upon this account. He had a Son of his own, the Lord Jesus.... so that God does not adopt out of necessity, but pity."
Well, we know that is not totally true. We know that many parents adopt children even though they have their own children. I read one statistic where one half of the adopted children are the only child in that home. Thus, there are still many parents who adopt out of need. God still adopts out of love and mercy and grace. He has his own Son, yet he still adopts.
2) When a man adopts, he adopts but one heir, but God adopts many. Most adoptive parents only adopt one or two children. God? Well, he adopts millions, if not billions. Thomas Watson uses Hebrews 2:10, "In bringing many sons to glory..." Parents adopt just one or two children because of limited resources. God's resources are endless, so he can adopt children endlessly.
3) Man when he adopts does it with ease. It is but sealing a deed and the thing is done. But when God adopts, it puts him to a far greater expense...It cost God the death of his natural Son, to make us his adopted sons.
Yes, there is a lot of paperwork and often quite a bit of expenses to adopt a child these days. But there is no doubt God had a higher expense. His own Son had to die in order for people to be adopted. What irony! What love on behalf of God!
4) Man, when he adopts, settles but earthly privileges upon his heir, but God settles heavenly privileges- justification, glorification....When God makes any his children he stamps his image upon them. This is more than any man living can do. He may adopt another, but he cannot alter his disposition...but God in making of his children fits them for sonship. He prepares them and sanctifies them for this privilege. He changes their disposition.
Most children become like their parents. However, that is not true with adopted children. Many have already developed their personality before they meet their adoptive parents. Their personality usually does not change much. However, the more we hang out with the Father, the more we become like him. Our personality becomes more like our Father.