The way into the holy of holies had been barred to the Israelites for centuries. Only the high priest was allowed to enter, and even then he could only go in on one day of the year, the Day of Atonement.
Assurance is given to us at the start of the Christian life with the gift of faith. We receive an inner certainty. As you hear the gospel, it becomes obvious and clear that this is true. Only God can provide and he has done so. But faith has to be exercised. It is true to you and you respond, but you must continue to exercise boldness. It is the opposite side of the coin that has faith on one side: to be given conviction carries assurance.
What is opened up to every child of God is an intimate audience with their heavenly Father. Although God is seated on the throne of his glory, and from there rules over all creation and directs the unfolding of all his will, we must not draw back in fear. We must come boldly to praise and thank him, and to make known our petitions. We must come to the place where his holiness would have destroyed us without the saving work of Christ.
Of course believing Israelites did not starting crowding into the holy of holies in the earthly temple. That would still have been barred to them by the priests. But they entered the true holy of holies and there they had fellowship with God.