Even though the world rejected Jesus, the Spirit characterized by truth would bear witness that Jesus was the Son of God (cf. John 14:16-17; John 14:26). He would do this after He came on the day of Pentecost. After that, the disciples would also testify, similarly empowered by the Spirit. The basis of their testimony would be their long association with and intimate knowledge of Jesus (cf. Acts 1:21-22). [source][source][source]
These verses explain how the conflict between Jesus and the world would continue after He departed to heaven. The essence of the conflict would continue to be who Jesus is. [source][source][source]John 15:26 also contains a strong testimony to the deity of the Holy Spirit whom Jesus described as proceeding from the Father as He had done (cf. John 14:26). [1] It refers to all three members of the Trinity and reveals something of their functional relationships to one another. "The beginning" is the beginning of Jesus" public ministry. [source][source][source]