Paul is willing to let the penny drop and one place he does is that the indicator that a person is God’s is if they have the Spirit — and one knows if one has the Spirit by the “witness of the Spirit with our spirit” (Rom 8:16).
First, Spirit indwellers call God “Abba, Father.” (One could get into some kind of debate here about whether or not this is appropriate in our day and age, but I have not found that debate to be anything other than a distraction from what both Jesus and Paul say: God is to be addressed as Father because the word “Father” says something true about God. God is not male; but God is in some ways “like a father” so “Father” is appropriate.)
Second, calling God “Abba, Father” indicates that the Spirit is present. It’s a circle. So be it. It’s true, Paul says, so run the full circle.
Third, calling God “Abba, Father” indicates a personal, intimate relationship with God. (No footnote here to Jeremias or anyone else; it’s an old footnote.)
Fourth, as Wright sums this up: “The Spirit’s very own self comes alongside our human spirit to bear witness that we are God’s children” (594).
Subjective? You bet. Possible to be mistaken? That too. Still true? Indeed.