that God was reconciling the world to Himself in Christ, not counting men’s trespasses against them. And He has committed to us the message of reconciliation. 2 Cor 5:19 BSB
that God was in Christ reconciling the world to Himself, not reckoning their trespasses to them, and having put into us the word of reconciliation. BLB
What DO we know?
- God was reconciling the world to Himself.
- we were reconciled to God by the death of his Son Rom 5:10
- All this is from God, who through Christ, reconciled us to Himself.
So God is the one reconciling, and it is through Christ - through his death.
The question was asked, In what sense was God in Christ?
Christ, Jesus, was doing things God could not do - like die. God needed a Lamb to offer for the sins of the world. His son, Jesus, was willing, but he could not do it alone.
Jesus stated that he could nothing of himself^. Clearly this needs to be clarified. He could do anything any other man could do, like lie, steal, murder. So what is it that he could only do with his Father's help?
God was always IN Christ via His spirit, holy at birth, given the spirit at baptism, and being 'heavenly' he was uncorrupted by sin and did not desire to sin, doing right and good was his default. However he still had his own will and this differed from his Father's will - noted in a few verses for our instruction. It was this difference that evil sought to exploit through persistent and extreme temptation.
So Jesus needed the Father's continual presence within him to avoid falling for any temptation - always, eventually, aligning his will with the Father's - not by a simple arbitrary decision, but in every need, he prayed for God's provision. His foe was spiritual and he needed the highest power in him to overcome the power of the world. Just as we do-
You, little children, are from God and have overcome them, because greater is He who is in you than he who is in the world. 1John 4:4
So, we could understand the verse as -
God was, in Christ, reconciling the world to Himself
or as the BSB has the focus already on what God is doing.
God was reconciling the world to Himself, in Christ
The verse in focus is not really about "God in Christ". It is firstly about what God is doing and then, who through.
This is no different to what we read of the logos. God created - through the logos. The logos does nothing on its own, just as Jesus does nothing on his own. God was IN both expressions of Himself.
God was IN Christ in every conceivable way - Jesus is God's logos! And Jesus, the human logos, allowed God to accomplish in him what could be done no other way. The fullness of God did indeed dwell in him.
[The noted ESV even has an appropriate comma missing. "that is, in Christ(,) God was reconciling the world to himself". I've no concept of what they mean by 'Christ God' - it is an unnecessary distraction.]
^ John 5:19, 30, 8:28