"atonement" means "reconciliation. Jesus reconciled the world to God through His death on the cross. Every human being has the potential to be saved if he/she chooses to accept this gift. That is the perfection of the completed atonement. There is nothing more to be done to make this atonement a finished "product." No one need be lost.
But just because the perfect atonement was achieved on the cross does not mean that everybody will automatically be saved.
"But as many as received him, to them gave he power to become the sons of God, [even] to them that believe on his name." John 1:12
"And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house." Acts 16:31
"And before him shall be gathered all nations: and he shall separate them one from another, as a shepherd divideth [his] sheep from the goats. And he shall set the sheep on his right hand, but the goats on the left. Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world: For I was an hungred, and ye gave me meat: I was thirsty, and ye gave me drink: I was a stranger, and ye took me in: Naked, and ye clothed me: I was sick, and ye visited me: I was in prison, and ye came unto me. Then shall the righteous answer him, saying, Lord, when saw we thee an hungred, and fed [thee]? or thirsty, and gave [thee] drink? When saw we thee a stranger, and took [thee] in? or naked, and clothed [thee]? Or when saw we thee sick, or in prison, and came unto thee? say unto you, Inasmuch as ye have done [it] unto one of the least of these my brethren, ye have done [it] unto me. Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels: For I was an hungred, and ye gave me no meat: I was thirsty, and ye gave me no drink: I was a stranger, and ye took me not in: naked, and ye clothed me not: sick, and in prison, and ye visited me not. Then shall they also answer him, saying, Lord, when saw we thee an hungred, or athirst, or a stranger, or naked, or sick, or in prison, and did not minister unto thee. Then shall he answer them, saying, Verily I say unto you, Inasmuch as ye did [it] not to one of the least of these, ye did [it] not to me. And these shall go away into everlasting punishment: but the righteous into life eternal." Matthew 25: 32-46.
"But he answered and said, Verily I say unto you, I know you not." Matthew 25:12
So apparently, the world has been given time in which humans, in their brief lifetimes, can make their decisions as to whether to accept salvation or not. And that is what the atoning ministry is about -- administering the benefits of the perfect atonement.
What are the benefits of the perfectly finished atonement? It provides forgiveness of sin and power imparted to overcome.