For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:
And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord:
for all shall know me, from the least to the greatest.
For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more.
In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away.
The assurance is He writes his covenant in your heart. If you believe scripture is the inspired word given of God, its words confirm that knowledge and both parallel. As a Baptist preacher said once, "You just know that you know.."
Regards...