[aMorePerfectUnion]

What makes you think you understand "the source"?

"But when He, the Spirit of truth, comes, He will guide you into all the truth;
 for He will not speak on His own, but whatever He hears, He will speak; 
and He will disclose to you what is to come."



- Jesus

[Angelino97]

According to the Catholic Church, in the verse that you cite, Jesus is promising to send the Holy Spirit to guide the Catholic Church's leaders (Peter being the first Pope), who will in turn guide the people.

That verse doesn't mean that any lay person can pick up a Bible and interpret it for themselves, relying solely on the Spirit. The Spirit guides you to the Church.

The Ethiopian eunich did not rely on the Spirit to interpret Isaiah for him. Instead, the Spirit sent Philip (the Church) to teach the eunich.