The same way one would establish what a priest is saying, since there could be (and are) many interpretations of what he says.
So the same way we might all hear God's Word and understand it in our own
way, is the very same way we'd all understand it if He were here in the flesh
to 'speak aloud' to us, face to face.
We'd all come away with our own understanding.
IOW, God is 'speaking to us' in written form with Scripture
just as He'd 'speak' to us if face to face.
Quite similar to the bishop might say something to us face to
face or in written form.
Can we understand the one man but not the other?
But God, on the other hand,
(Why did He give us His Scripture?)
is able to "reveal" Scripture to us, so that
we CAN know what He wants us to.
(whereas our bishop, may or may not be understood
His church is 'built' on revelation.
King James Bible (Cambridge Ed.)
Then opened he their understanding,
that they might understand the scriptures,
Awesome, isn't He
"Were not our hearts burning within us while he
talked with us on the road and opened the Scriptures to us?"