Because, as you say, it is mentioned only in the Epistles. And what did the Christians teach before Paul came up with this term, before he wrote the Epistles?
Were they told they would be sanctified, and by whom were they told? The term does not appear in the Bible save for the Epistles, so no other Apostle, who actually knew Christ in Person, taught it or knew it for that matter.
Did the rest of the Apostles not teach sanctification? If not, and they don't mention it, then is it Paul's Christianity, and is it not then Paul defining Christianity alone in this particular case?
Does that mean the Apostles didn't know it yet or simply that the rest were ad odds with Paul?
How can you jump to so many conclusions without getting winded?
How can anyone be so gullible and not choke?
***Because, as you say, it is mentioned only in the Epistles.***
I DIDN’T say it was ONLY in the epistles. You said it wasn’t even in the Bible. I showed you one place where it is. Others:
Direct my footsteps according to your word; let no sin rule over me.
How can a young man keep his way pure? By living according to your word.
Your statutes are my delight; they are my councilors.
My soul is weary with sorrow; strengthen me according to your word.
Turn my eyes away from worthless things; preserve my life according to your word.
And much much more. Sanctification is rampant throughout the Bible. All these came from one psalm.
***The term does not appear in the Bible save for the Epistles, so no other Apostle, who actually knew Christ in Person, taught it or knew it for that matter.***
Then why does 1 Peter (who was not Paul) mention sanctification in verse 2 of chapter 1?
Keep jumping.