OUR LORD WHO ASSURES US
In the book of Hebrews Chapter 11 verse 19 - 23, it says:
Therefore, brethren, having boldness to enter the Holiest by the blood of Jesus, by a new and living way which He consecrated for us, through the veil, that is, His flesh, and having a High Priest over the house of God, let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience and our bodies washed with pure water.
Let us hold fast the confession of our hope without wavering, for He who promised is faithful.
Here, the scriptures are telling us that we can have full assurance of our right standing with God because of the sacrifice of Jesus which washes us and through the sprinkling of His blood, we are cleansed of our evil conscienses and our bodies are washed by pure water.
Further to that, the Lord who provided these instruments of sanctification is also faithful and will complete the good work of that He has began in us. - Philippians 1 verse 6. (Paraphrased)
The core of assurance comes from truly knowing the most excellent Jesus Christ and understanding the wonders of His being and who contains all the hidden perfection and wisdom of God.
Colossians chapter 2 verse 2 describes it like this:
that their hearts may be encouraged, having been knit together in love, and attaining to all the wealth that comes from the full assurance of understanding, resulting in a true knowledge of God’s mystery, that is, Christ Himself, in whom are hidden all the treasures of wisdom and knowledge.
Let us rest in the assurance of the Lord's salvation and be able to say, "Surely I have composed and quieted my soul; Like a weaned child rests against his mother, My soul is like a weaned child within me." -Psalm 132 verse 2.
Bless the Lord.