It is both accomplished by HIS work, before you are saved, but then He gives you increase in knowledge to understand it.
You do not have this understanding upon initial salvation...in other words this understanding is NOT WHAT SAVES YOU...IT IS WHAT SEALS YOU:
John 6:27 Labour not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you: for him hath God the Father sealed.
They confuse receiving the Holy Spirit with having the Holy Spirit sealed, which is two different things.
It is the meat that seals us or confirms us, so that we can no longer be tossed to and fro.
We are all called to 'test' the spirits, to see if what we believe is truth, and we must do so by being a Berean, and rightly dividing the word of God, which can only be done, IF we hear HIS voice over men..
We all have the freewill to place the voices of men, above and beyond HIS, which is why many are apostate.