Bavinck on the foundations of theology

"Holy Scripture is the external instrumental efficient cause of theology, and divine revelation also requires the internal illumination of the Holy Spirit. We thus identify three fundamental principles for theology: God is the essential foundation (principium essendi); Scripture is the external cognitive foundation (principium cognoscendi); and the Holy Spirit is the internal principle of knowing (principium cognoscendi internum). The foundations of theology are thus trinitarian: The Father, through the Son as Logos, imparts himself to his creatures in the Spirit." Harman Bavinck, Reformed Dogmatics