Bear with me a bit. He atoned for all sins that had ever occurred and would ever occur. That atonement opened the gates of heaven. That, in no way, guarantees us heaven. We have to persevere to the end in faith, performing works of charity, for we will be known by our charity and judged according to our works.
Now, as to suffering, our suffering may be added to that of Christ, since we are, in the mystical sense, actual parts of His Body - that is why the Eucharist nourishes us - it nourishes His Body so that His mission continues unto the end of the age. For this same reason, the Eucharist does not spiritually nourish animals, but only we who are made in the image and likeness of God and have been incorporated into Christ’s Body.
Even though he opened the gates of heaven, His mission continues, and we are grafted into His Body. This is the gap, the chasm which must be understood. We are continuing His mission of suffering and redemption in each generation. Thus, just as He suffered for the sake of His Church, so can we also suffer for the sake of His Church.
A popular but erroneous view is that He is in heaven and we are in His waiting room here. Partially true, but we must act to build the Church. In that building up, in that mission, we will suffer.
Just as Christ offered His suffering to the Father for our sake we, being incorporated (corpus = body) into both His Body and His mission, can offer our suffering to the Father for the sake of others - the Church. God knows who will be saved and who will not. For that reason, we must assume that all have the potential to be saved.
Therefore, offering our suffering for the conversion of sinners is part of that “eventual” Church - the Church which does not yet exist, but is being built by our faith and perseverance in our mission. Salvation is on-going in each age, and while the gates of heaven are open, no one carries us through them. God’s grace gets us there, but we must cooperate with that grace in the building up of God’s Church.