Your post is not clear at all. What do you mean when you say this;
Where does the text say anything about being "adopted in a certain way"
Where does the text say that?
What is that certain way?
Where does it say...how man will be saved?
He could have adopted us by living a good moral life.
He could have adopted us by Church membership.
He could have adopted us by random choice.
He could have adopted us by by our good outweighing our bad.
But He chose to adopt us by Jesus Christ according to His good pleasure.