The Mystery and Hope of Adoption as God's Children

I am basking in the mystery and promise of Eph 1.5 (early) this morning. God decided in advance to adopt us into his own family by bringing us to himself through Jesus Christ. This is what he wanted to do, and it gave him great pleasure. These words are fully of mystery and hope. What does it mean that God decided this in advance? How? Paul doesn't bother to dwell there, probably because God has given no direct answer to this question. It is a mystery that he has allowed to remain a mystery. I am grateful for God's action in advance, meanwhile I do not understand it.

On the other hand being adopted into his own family I do understand, at least partially. This fact gives me (and every follower of Christ) great hope because our lives have purpose and meaning, and there is an end, or purpose to all of this. I, We !, those who follow Christ by faith, are part of God's family. Incredible.