Jesus spoke more on the reality of Hell than anyone else in the scriptures.
Concerning this, He told us that He is the way, the truth, and the life, and no one comes to the Father, but through Him. In 1 John, we are instructed that whoever has the Son also has the Father, but whoever does not have the Son, doesn't have the Father either. This excludes all other 'paths' to Heaven. As far as severity of punishment, Jesus put it this way,
The Father loves the Son and has given all things into his hand. Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him.
John 3:35-36
and also
"And that servant who knew his masters will but did not get ready or act according to his will, will receive a severe beating.
But the one who did not know, and did what deserved a beating, will receive a light beating. Everyone to whom much was given, of him much will be required, and from him to whom they entrusted much, they will demand the more."
Luke 12:47-48
Those who know more of what is expected will receive a greater punishment than those who did not know. For example, there are a great many people who go to church week after week, hearing the gospel, or coming to forums and websites where the gospel is taught, and they read of their need to repent, over and over, and reject it time after time. Their judgment will be extremely severe as opposed to others who did not have the same revelation given to them time after time.
It's a very dangerous thing for those who only seek to confront christians, with the intent to harass, and are met with scripture in return. They will be held accountable for every instance they reject the truth given to them.
Paul describes it as such when speaking of those who reject God's kindness by not repenting, and are thereby stradily storing up wrath against them, and they will not be given eternal life, but rather the wrath and fury of God...
Or do you presume on the riches of his kindness and forbearance and patience, not knowing that Gods kindness is meant to lead you to repentance? But because of your hard and impenitent heart you are storing up wrath for yourself on the day of wrath when Gods righteous judgment will be revealed.
He will render to each one according to his works: to those who by patience in well-doing seek for glory and honor and immortality, he will give eternal life; but for those who are self-seeking and do not obey the truth, but obey unrighteousness, there will be wrath and fury.
Romans 2:4-8