From Reformed Theologian and Missionary Lesslie Newbigin in Sin and Salvation (1956) (you can read the whole book online in PDF from an external website here):
In the cross God’s judgment and His salvation are revealed, but they are not fully consummated. It is there revealed that the whole human race is under the condemation of God's holiness. But God has not carried out that sentence of condemnation. He holds it back, so to say, in order to give men time to repent. He sends out His Church to tell all men the gospel, in order that they may repent and be saved. His will is that all should be saved. He does not will the destruction of any soul. Some Christians have taught that God created some men for the purpose of destroying them, but that is quite false teaching. It is based upon a misunderstanding of certain passages in Scripture, but it is certainly not the teaching of the Gospel.If God wills that all should be saved, does that mean that all will be saved? We cannot say that. We know that God has given men freedom to choose good or evil. We cannot say that it is impossible that men should finally choose evil. Christ has given us many terrible parables in which we have a picture of men finally cast out of the light and love of home into the outer darkness. He has also used the name Gehenna to describe this final destruction. Gehenna was the name of the valley where the rubbish and filth of Jerusalem was deposited. It was a place where there were always fires burning. It was a symbol to Him of the possibility of men becoming finally useless and fit only for burning. We cannot exclude this possibility from our minds, if we wish to remain true to His mind.The disciples once asked Jesus: ‘Lord, are they few that be saved?’ [Luke 13.23] Jesus answered: ‘Strive to enter in by the narrow door, for many, I say unto you, shall seek to enter in and shall not be able’. Jesus does not answer our theoretical questions about Hell. But He bids us recognize that the door into life is narrow, and that it is possible, and indeed terribly easy to miss it. In the end this is certain: that what opposes the love of God must be done away. We believe that God wills to knit together all His created world in one common salvation, in which the glory of His love will be perfectly revealed and reflected. We have seen in Jesus that this involves a judgment upon the whole human race as it is now. We know that He has given time for men to hear and believe the gospel, to repent, and to return to Him. But at the end we cannot deny the possibility that men – even the majority of men – may be left outside. If they are left outside, it will be because – like the elder brother in the parable – they are not willing to share the Father’s fellowship on His terms.His invitation is to everyone. ‘He that cometh unto me I will in no wise cast out.' [John 6:37] But when we begin to speculate about the question of eternal loss we are quickly in regions where we do not know the answer. We can only give heed to the words of our Lord: ‘Strive to enter in by the narrow door’.
You can read the whole book online here.