This is one thing that I have found turns a lot of people away from Christianity. So I would like to start this discussion so that we can analyze this subject together.
God does not grade on the curb—good or bad. And that is what is unique to Christianity as opposed to all other religions and it is what many don’t agree with. There will be good people in hell, and so called bad people in heaven. For God’s grade is Himself, which is perfection and anything that falls short of Himself is too much wickedness and does not meet His holy standard. Isaiah 64:6 says, “All of us have become like one who is unclean, and all our righteous acts are like filthy rags; we all shrivel up like a leaf, and like the wind our sins sweep us away.” Isaiah says that on our best day we fall short of a holy God’s demands. No matter how much we pray, give to charities, shun evil, seek good, etc.—on our best day, our good acts fall short of a holy God.
To think about fairness we have to consider God’s nature of holiness and perfection and we have to consider the fact that God knowing that mankind falls short of His righteous demands made the provision to send His Righteous, perfect and holy Son to come to Earth and die as a sacrifice for mankind's sins that propitiates all of God’s demands forever. This has nothing to do with how good or bad we live, but everything to do with accepting God’s payment of sin that He provides us through faith. Once we cross over from death to life righteousness is demanded of us because God disciplines His own when they do evil, but they are eternally bought and will always be saved from hell. Once a person accepts Jesus, that person is eternally saved and sealed by the Holy Spirit until the Day of Redemption, so we cannot break that seal by any act we do.
Another thing we need to consider is that the bible shows that both heaven and hell are not equal for everyone there. Concerning heaven, Matthew 5:19 says, “Anyone who breaks one of the least of these commandments and teaches others to do the same will be called least in the kingdom of heaven.” We see from this verse that some will be “greater” than others and not all have equal rank in heaven depending on how they lived on Earth. Other verses show that we will be rewarded crowns based on how we lived. And concerning hell, Luke 10:12 says, “It will be (so that’s future tense) more bearable on that day for Sodom (but wait, how can it be future tense if Sodom was destroyed in the past? Because this is when Sodom is thrown into the lake of fire, which is yet to come. So this is speaking of hell.) than for that town.” Jesus said that the town that rejected Him when He was physically in their presence will have the thermostat turned up greater on them than Sodom will have on them. This shows that hell will not be equal for everyone, but according to how they lived on Earth.
By all means, God is a just God. Romans 2:6 says, “God will give to each person according to what he has done.” But as far as heaven and hell goes, that is not about works of righteousness or evil at all.
And lastly, there is the issue of babies. The bible says that the blood of Jesus covers and saves all babies who die as babies. 2 Corinthians 5:18-19 states, “All this is from God, who reconciled us to himself through Christ and gave us the ministry of reconciliation: that God was reconciling the world to himself in Christ, not counting man’s sins against them.” This says that God reconciled the entire world’s sins in Christ. That should raise the question: if this says that God reconciled the world’s sin in Christ, then how come John 3:16 says, ‘For God so loved the world that he gave his only begotten Son that whoever believes in him shall not perish, but have eternal life.”? That seems to contradict. It doesn’t contradict. Putting the verses together reveals that Jesus’ work at the cross purified the sin of all of mankind, but to access it is by faith and accepting His payment. But babies and infants are unable to believe because they are too young to understand what Jesus did for them. So since Jesus purified the sins of the world, in order to not be a Christian, one has to opt out through rejecting the payment Jesus did for us. We cannot reject until we are at the age of understanding. This includes mentally challenged people who are not capable of rejecting Christ. Jesus purified their sins too and they are incapable of rejecting Him as their Lord.
These are all things we should consider before answering the question: is hell fair.