Honestly, there's no "proof" for Universalism. Heck, there's no proof for much of anything. But, there are some very strong arguments for Universalism from philosophical, theological and scriptural points of view. I'll be getting into more of this over time. Meanwhile, here is a very simple case for Universalism.
Here is a very simple one:
- G-d is all powerful (He will accomplish what He desires)
- G-d is all loving (He desires that none should perish)
- Some will suffer eternal torment.
Read each of the statements. You can agree with two of them, but not all three at the same time. Now, you don't have to agree with any of them. Calvinists agree with 1 and 3. G-d's grace is inescapable, but only for those who He predestines. He doesn't love those He hasn't chosen and they have no hope of salvation because He doesn't will it. So, Calvinists will disagree with number 2. Arminians agree with 2 and 3. They disagree with 1. G-d either cannot or will not overcome man's "free will" and chooses to let some perish or can't stop them from perishing. Universalists believe that G-d not only wills that none perish but that He can (and has) provided) salvation to all.
Ironically, Calvinists and Arminians seem to get along pretty well with neither camp calling the other heretical. However, when you start touting the dangerous idea that G-d is capable of loving and saving all of His children, look out. Why is this?