Apart from the Scriptures, what proof do we have that the Holy Spirit dwells in believers?

Let us look at it in a different light. What are the Scriptures? They come after the oral preaching; they record in writing what was said when the talking was taking place. In the first century you had the apostles preaching to the bishops, priests, and people of Jerusalem and Judea, and the first part of the New Testament was not written until around 57 AD with Romans, so nearly half a century goes by with nothing written down, and it takes a good century to complete all the writing. So where does Scripture get its information that the Spirit indwells believers? It was already known beforehand through oral transmission, and then Paul and Peter wrote it down, guided so that their words would be without error. So you have two witnesses: the oral witness, as 2 Thessalonians 2:15 speaks of, and the written witness. And then you have the confirmation of both by the actual events, beginning when Jesus breathed on the apostles in John 20, then Pentecost, then the miracles, the speaking in tongues, and the Spirit active and indwelling. So the proof is not Scripture alone; it is the oral tradition, the written record, and the confirming events all together.