The Day of Atonement Symbolized the Pre-Advent Judgment. The Day of Atonement was a judgment day. It put a seal upon the forgiveness provided throughout the previous year. It also resulted in God's rejection of individuals who had refused to confess their sins.
Since the people's sins had already been forgiven, why was the Day of Atonement judgment necessary? This judgment was God's way of demonstrating that He finally rejects professed believers who refuse to maintain a relationship with Him. He finally provides a seal of approval for those who, by His grace, have forsaken sin and maintained their heart union with Him.
God mercifully gave one final day on which the individual whose heart was not right could come into a covenant relationship with Him. On this day, sinners were given opportunity to be forgiven and to enter into a new and victorious heart union with their Lord.