This is the way a young child would address his father in Paul's home language, but not in the language of the Galatian readers. To keep the sense of a foreign language, translate this as a word that sounds as much like "Abba" as your language allows.
"sent his Son's Spirit to show us how to think and act"
## who calls ##
the Spirit is the one who calls
## sons … son … son ##
Paul uses the word for male child here because the subject is inheritance. In his culture and that of his readers, inheritance passed most commonly, but not always, to male children. He was neither specifying nor excluding female children here.
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