Consider the Troubled Israelite

Let us imagine that we stand near the altar in the temple court, as a troubled Israelite comes with his sacrifice. He leads a goat along to the place of the oblation. The priest examines it carefully, and finding it without any outward blemish he commands it to be slain. The offerer himself puts the knife to its throat, after laying his hand on its head. Then it is flayed and cut in pieces, and all its inward parts carefully inspected. Pronounced perfect, it is accepted and certain parts are placed upon the fire of the altar. The blood is sprinkled round about the altar and upon its four horns, after which the priest pronounces absolution, assuring the man of his forgiveness.
This was but "a shadow of good things to come," and could not actually put away sin. That unblemished animal typified the sinless Saviour who became the great Sin Offering. His blood has made full and complete expiation for iniquity. All who come to God through Him are eternally forgiven.
If the Israelite sinned against the Lord, on the morrow he required a new sacrifice. His conscience was never made perfect. But Christ's one offering is of such infinite value that it settles the sin question eternally for all who put their trust in Him. "By one offering he bath perfected forever them that are sanctified." To be sanctified in this sense is to be set apart to God in all the value of the atoning work and the personal perfections of Christ. He is Himself our sanctification. God sees us henceforth in His Son.
Is not this a wonderfully precious truth? It is something man would never have dreamed of. God alone devised such a plan. He who believes His testimony regarding it has full assurance of faith.
He does not know he is saved because he feels happy. But every true believer will be happy to know he is saved.
Confidence based upon an emotional experience would leave one in utter bewilderment when that emotion passed away. But assurance based upon the Word of God abides, because that Word is unchangeable.