If we are all a unique creation of the Father, and if His sacrifice was once, and for all, then why are we not all children of God?
In my opinion, I think you are pretty close in your interpretation. I would add that generally speaking, we have opportunities of grace, and a desire to do good, as inspiration from God.
Should an atheist seek to feed the poor and bury the dead, we should rejoice and commend them. They are responding to God’s call to charity.
Let us hope in doing so, the atheist will be more open to the reality of God and draw closer to Him.
We would all do well to remember that who goes to Heaven is a Management decisions. Those of us on Earth are merely in Sales.
Because I'm pretty sure both Scripture and Church Sacred Tradition make it clear that it is in Baptism that we become children or sons of God. We may all be made in the image and likeness of God but that doesn't necessarily mean we are all children of God. Once again, it is in accepting His Son that we become part of God's Family. Atheists (and others ) have not done that (yet).