Romans 8:16

16HimselfαὐτὸtheτὸSpiritπνεῦμαbears witness withσυμμαρτυρεῖtheτῷspiritπνεύματιof usἡμῶνthatὅτιwe areἐσμὲνchildrenτέκναof Godθεοῦ.

The Spirit Himself testifies with our spirit that we are children of God,