With this phrase Paul hints at the close relationship that exists between the persons of the Father, Son, and Spirit. The Spirit is the Spirit of the Son because he proceeds from the Son and the Father (John 15:26) and glorifies the Son (John 16:14).1
6 Et parce que vous êtes fils, Dieu a envoyé dans nos cœurs l'Esprit de son Fils, lequel crie: Abba! Père!