Before his death, Jesus prepared his disciples. "I'm going away," he told them, "but, I am sending you the Advocate."
"This helper will testify to the world and to you."
In Preaching through the Christian Year B, the always-excellent Fred Craddock says:
The Spirit will bear witness to Jesus just as the apostles who shared his earthly life bear witness.... The church is not left with only the promptings of the Spirit, opening wide the door to ... all sorts of nonverifiable claims by inspired speakers. Neither is it the case that the church is left with just the reports of men of antiquity. Rather, the church has both the Spirit and the tradition, and the gospel is located where these two intersect.The Spirit came to the original disciples and continues to come to disciples. We are left with this reassurance and the implied problem of discerning which spirits come from Christ.