Universalism believes it would be against God's perfect character to allow anyone to be "lost" for all eternity either in an ever-burning Hell or in the permanent destruction of their life. The Eternal's love toward us, universalism argues, is so great that he will pursue every human, for however long it takes, until they are in his kingdom.
Some of the many Bible verses used to support universalism are John 3:16 and 12:32, Romans 5:18, 1Corinthians 15:22, Ephesians 1:10, Colossians 1:20, 1Timothy 2:4 and 4:10, 2Peter 3:9 and 1John 2:2 to name a few.
The early Christian scholar Origen (c. 185 to c. 253 A.D.) is one of the earliest proponents of this doctrine. He not only taught the eventual salvation of all humanity, but also promoted the concept that even Satan the devil (and the demons?) would eventually be reconciled to God.
In contrast to universalism is conditionalism (conditional immortality). This doctrine teaches that humans are mortal (no immortal soul) and, as such, can only live forever if they believe in Jesus and receive eternal life as God's gift. Unlike universalism, where 100% of humanity is ultimately saved, conditionalism states some humans will not receive immortality.
Closely related to conditionalism is the teaching of annihilation. This belief states that someday the unsaved, which have no inherent immortality, will be destroyed and cease to exist instead of being punished forever in an ever-burning Hell.
Universalism assumes the Eternal will spend whatever amount of time is needed to convince all humans to repent. Scripture, however, indicates he has a timetable. All humans, up to Jesus' return, who never fully understood the truth will be resurrected back to a flesh-based life in the Great White Throne Judgment (Revelation 20:11 - 13).
The Bible reveals that this massive group of people will have their minds open to God's way (Revelation 20:12 - 15). They will then be given (possibly) up to 100 years of life and allowed to choose either to repent and build righteous character or reject God (Isaiah 65:17 - 20).
God has determined that a century of Jesus ruling over the earth and teaching all those in the second resurrection (Revelation 20:4) is enough for people to make their choice and for their eternal fates to be determined.
In a further refuting of universalism, the Bible teaches that some people will choose to reject, willfully, salvation (the unpardonable sin). Their choice will be honored by God who will mercifully end their self-torturing existence.
The one who believes in the Son has everlasting life; but the one who does not obey the Son SHALL NOT SEE LIFE, for the wrath of God remains on him (John 3:36, HBFV throughout).
FOR IT IS IMPOSSIBLE for those who were once enlightened, and who personally obtained the heavenly gift, and became partakers of the Holy Spirit . . . If they have fallen away, to renew them again unto repentance; seeing that they are crucifying the Son of God for themselves, and are publicly holding Him in contempt (Hebrews 6:4 - 6).
For if we willfully go on sinning after receiving the knowledge of the truth, THERE IS NO LONGER ANY SACRIFICE FOR SINS, but a terrifying expectation of inevitable judgment and of fierce fire, which will devour the adversaries of God (Hebrews 10:26 - 27).
And if anyone was not found written in the book of life, he was cast into the lake of fire (Revelation 20:12, 15).
Additional Biblical verses that refute the universalism notion that 100% of humans will receive salvation are Psalm 37:9 - 10, 145:20, Isaiah 5:20 - 24, Obadiah 1:16, Matthew 3:11 - 12, 5:22, 13:40 - 42, John 15:6, Romans 2:5 - 9, 6:23, 2Thessalonians 1:7 - 9, 2Peter 3:7, Hebrews 12:25 - 29 and Revelation 21:7 - 8, 27.