Friedrich Schleiermacher (1768—1834), the father of modern theology and liberal Protestantism was one of the first theologians to write that is it is not possible for people to be eternally blessed (in heaven) when they know that others are in eternal torment (in hell.) Before Schleiermacher theologians throughout church history had argued that the blessedness of people in heaven would be increased by their knowledge of people who are tormented in hell, as if their unending torments glorified god as a just judge. (For example, read St. Augustine's Book XXI on hell in The City of God). Schleiermacher was among the earliest and most important theologians to admit that the eternal torment (in hell) calls into question the goodness of god and the possibility of eternal blessedness (in heaven).

6 Reasons Why Eternal Torment makes Eternal Blessedness impossible

In Schleiermacher's Christian Faith §163 there is an addendum called "Regarding Eternal Damnation" where Schleiermacher argues that the knowledge of other people in eternal torment would make it impossible for anyone to be eternally blessed because: 

1) the eternally blessed will know that others are eternally tormented (even if those in torment cannot be seen),

2) the eternally blessed will remember their loved ones who have entered into eternal torment,

3) the eternally blessed will no longer have hope for those in eternal torment like in former times,

4) the eternally blessed will have a greater sense of mercy and compassion for others who are suffering and lost,

5) the eternally blessed will remember that they were once in the same state as the eternally tormented, and will grieve that they were chosen and others were not, and lastly that

6) the eternally blessed will know that the restoration of all things has not occurred, and god is not "all in all" as the Scriptures promised, so long as anyone is in eternal torment.

Therefore Schleiermacher concludes that eternal blessedness is not possible without universal reconciliation of all (i.e. Universalism). 

Regarding Eternal Damnation

I derived my list of six reasons from Schleiermacher's "Regarding Eternal Damnation", and here are the quotations (in loc.) that I used to create this list with commentary. The following quotations are one continuous selection from this addendum that I've divided up in order to provide commentary.

Schleiermacher begins be explaining that people experiencing eternal blessedness (in heaven) would have knowledge of people existing in eternal torment in hell, even if they were unable to see the others in torment:

If we now consider eternal damnation in its relation to eternal blessedness, it is easy to see that if eternal damnation exists, eternal blessedness cannot continue to exist. That is to say, even if the two domains were totally separate externally, such an elevated state of the blessedness, already regarded in itself, could not be co-joined with complete ignorance of others' lack of blessedness, even less so if that very separation were to be simply the result of a general judgment at which the two sides were present—that is, in which each side would also be aware of the other. [1]

Schleiermacher continues to argue that people entering the state of eternal blessedness would have a heightened sense of compassion, and would have more love and compassion for those who are suffering and for their enemies that would cause them more grief. Also, before a person enters eternal torment, there is still hope that eternal torment may be avoided somehow, but once a person is consigned to eternal torment, there is no longer hope for them, so that final state is more grief for the eternal blessed:

If we accordingly apply to the blessed some knowledge regarding the condition of the damned, this knowledge cannot be imagined to be devoid of any feeling for them. This is so, for if the perfecting of our nature is not to have retrogressed, this feeling must embrace the entire human race, and feeling for the damned must needs disturb the state of blessedness, all the more so in that, unlike every similar feeling in this life, it would not be tempered by hope. In this connection, however much we might bear in mind that if eternal damnation really exists, it must also be just and that God's justice must be included in people's beholding God, feeling for the plight of others also cannot be removed from that experience. Likewise, then, in this life as well we would rightly require a deeper compassion for suffering that is deserved than for that which is not deserved. [2]

Schleiermacher then says the eternally blessed would remember that they were no better than those who are now experiencing eternal torment, and would be grief stricken to know that they may have been in torment, and others who are no different than they are now in this eternal torment:

Further, suppose that remembrance of an earlier state, in which some of us were always united with some of those others in the same collective life, would somehow continue in us beyond deaths. In that case, our feeling for them would have to be all the stronger if in this earlier period there was a time when we were no more regenerate than they. Then, since in the divine government of the world everything is integrally conditioned by everything else, we also cannot hide from the fact that things worked out well for us that were conditioned by the very same arrangement of the world, by virtue of which such fortune did not fall to them. As a result, a piercing quality must be added to our feeling for them that cannot fail to strike us whenever we sense a definite connection between our advantage and the disadvantage of anyone else. [3]

Schleiermacher concludes that we should not assumed that the greater part of humanity would be finally condemned to eternal torment, or to conclude that Jesus's words indicated that Jesus believed that most of humanity would be condemned to eternal torment (like Augustine's believe about The Mass of Perdition):

Thus, viewed from both sides, there are great difficulties in trying to envisage that the eventual outcome of redemption would be such that thereby some would have a share in supreme blessedness but others—and indeed according to the conventional notion the largest portion of the human race—would be irretrievably lost in a state lacking blessedness. In consequence, we should not cling to such a notion without decisive evidence that Christ himself foresaw this outcome in that fashion, and in no way do we have such evidence. Hence, we surely ought, at the very least, to grant equal right to that more moderate outlook of which there are also still some traces in Scripture, namely that by the power of redemption a general restoration of all human souls would eventually occur. [4]

Sources:

1. Friedrich Schleiermacher, Christian Faith: A New Translation and Critical Edition, trans. T. N. Tice, C. L. Kelsey, E. Lawler. ed. C. L. Kelsey, T. N. Tice, (Westminster John Knox Press, Louisville: 2016). Section 163. pp. 997-998. [P.S. 1] Addendum [to the Fourth Point of Doctrine]: Regarding Eternal Damnation"

2. Ibid.

3. Ibid.

4. Ibid.