If Christian-based Universalism is true, then it means Jesus atoned for everybody who ever lived. Since Scripture says we are justified by faith (Rom. 3:28; 4:1-5; 5:1; Eph. 2:8-9), we would also know that the Gospel is a necessity for people to believe because the Bible tells us that people are commanded to repent and believe the Gospel (Mark 1:15). The case of infants and the severely mentally challenged are not explained in Scripture, but we know that adults are commanded to believe. If that's the case and if Christian universalism is true, it should not occur that Jesus would prevent the belief of people in the Gospel or that He would desire that they not be forgiven. But, that is exactly the case.
"And as soon as He was alone, His followers, along with the twelve, began asking Him about the parables. 11 And He was saying to them, “To you has been given the mystery of the kingdom of God; but those who are outside get everything in parables, 12 in order that while seeing, they may see and not perceive; and while hearing, they may hear and not understand lest they return and be forgiven," (Mark 4:10-12).
Jesus clearly tells his followers that the reason He is speaking in parables is so people will not be forgiven. Clearly, it is His intent that they be judged instead of forgiven. Why would He do that if Universalism is true? If God wants all people to be saved, which is what Universalism asserts, why did Jesus speak in such a way that people would not be forgiven?