***Playing devil’s advocate, the sola scriptura answer would be “I KNOW — that’s God’s promise to me. Why don’t you believe and have the same belief? Christ died for us, so why doubt Him???”***
There are two immediate considerations. The first is that how does one know that one interprets Scripture correctly? The second is that how does one know that one’s belief is the same belief that Jesus gave to the Apostles.
In neither case, outside of the Church, does one have any assurance that their beliefs are correct or anywhere near correct. The Reverend Jim Jones, pre Kool Aid dispensing, no doubt absolutely believed that his faith was the correct one. The JWs believe. The LDS believe. And so on.
Jesus preached one church and one belief. The plethora of beliefs give lie to the odd notion that the Protestant Reformation, the Restoration and the subsequent buddings bring us correct beliefs.
Obviosuly some only believe he made them a promise. Calling "Lord, Lord," and claiming you believe is not enough. The Gospels are clear on that:
42 for I was hungry, and you gave Me nothing to eat; I was thirsty, and you gave Me nothing to drink;
43 I was a stranger, and you did not invite Me in; naked, and you did not clothe Me; sick, and in prison, and you did not visit Me.'
44 "Then they themselves also will answer, 'Lord, when did we see You hungry, or thirsty, or a stranger, or naked, or sick, or in prison, and did not take care of You?'
45 "Then He will answer them, 'Truly I say to you, to the extent that you did not do it to one of the least of these, you did not do it to Me.'
46 "These will go away into eternal punishment, but the righteous into eternal life." [Mat 25:41-46, NAB]
But the Prots tend to disreagrd the Gospekls a great deal in favor of Paul's epistles and even the OT.