If I may add to this conversation?
I'm not a Christian anymore, but I've grown up in a very Christian family, and as a preacher's kid I know my fair share of theology. Christianity is a very large and diverse religion with many different interpretations taken of it, but Annuil is right in that in the branch of Christianity I grew up in, and presumably in what Minna has turned to, it is only through faith in Jesus's death and resurrection that you can be saved and brought into God's kingdom - because every person sins and God is perfect, nobody is good enough for God, and thus only through trusting in Jesus's redemptive act of sacrifice (taking our punishment onto himself on the cross) can you be saved. Yes, this condemns everyone from any other religion. Personally, that's something I take issue with, and is one of the big reasons I left the religion - I don't believe it is fair, or just, to condemn people to hell for something in their nature that they cannot change. However, when you are looking at Minna's beliefs and the doctrine of many sects of Christianity, this is an aspect of theology that you cannot ignore. There is also the belief that scripture - the Bible - is inspired by God, and thus every book in the Bible is there because of God's will, not human decision or weird historical shenanigans (again, something I personally cannot bring myself to believe).
With this in mind, the extremely evangelistic outlook that many Christians such as Minna take starts to make sense - if everybody who doesn't put their faith in Jesus is condemned, then logically it follows that you should try and convert as many people as you can. I can understand why Minna wrote what she did in her comic, because it's shockingly similar to much of the other Christian media of questionable quality that I was fed as a child, and in many aspects it's similar to the beliefs of my family and the beliefs I myself once held.
However, even taking all this into account, if you look at the comic from an evangelistic perspective it is still fundamentally flawed. As I said earlier, it doesn't provide any good reasoning as to why Christianity is a religion you should follow - if you don't already at least nominally believe in the Christian God, then why should you care about the message Minna puts across? It's the evangelistic equivalent of whacking a starving man in the face with a sack of potatoes instead of feeding them an actual meal.
In addition, it neglects what I think are some of the most important parts of Christian doctrine - the commitment towards loving others in your society, regardless of their beliefs. Jesus didn't detach himself from the 'evils of our society. He ate with 'sinful' tax collectors and prostitutes, and acted with compassion and empathy towards marginalized people. Sitting slightly ironically beside the doctrine of original sin is the conviction within Christian theology that Jesus's sacrifice on the cross was the ultimate act of love for humanity, and the way which Christians are supposed to share Christ's message isn't by whacking people with badly paraphrased theology, it's by showing Christ's love to the people around them. The Christians I know who have done truly good things in the world have done it in the name of Christ's love. This comic is not showing love to anyone. The idea that in order to follow Christ you must detach yourself completely from the 'evil' modern culture is what leads to cult-like sects that genuinely harm the people within them - I've talked to people whose sects of Christianity treated everything about modern culture as evil, and it really, really messed them up. It certainly doesn't drive people to show love and compassion to those around them.
I also very much agree with Róisín and Azuki - fundamentalist Christianity and its extremely evangelistic attitudes have caused genuine damage to a large number of minority groups on its outside, as well as (within some sects) the people on the inside as well. I believe that people have the right to hold to whatever faith they want, but when your religion dictates that all other faiths lead to damnation, it's very easy to fall into the idea that all other cultures are inherently inferior to yours. There is a long history of Christians forcibly shoving their religion onto people from other cultures, and often forcibly shoving their entire culture along with it. I completely respect people's free choice to convert to another religion, including Christianity, but yeah. Attitudes like the ones Minna seems to be promoting have caused a lot of damage. Additionally, there are several parts in her dialogue that seem critical of modern culture becoming more inclusive of people of different beliefs, which is a yikes from me. Her story displays a pretty large persecution complex - the idea that because society is able to be critical of your religion, and other religions exist in the same space, that means you're being persecuted, which fosters an us/them dichotomy as people who hold that religion see outsiders as the enemy, and also trivializes the experiences of people who actually experience religious persecution.
As for me personally, I have to face with the fact that in less than a year's time, when I graduate high school and turn 18, I'm going to tell my parents that I'm both queer and no longer Christian, essentially damning myself to hell in their eyes (I really, really don't want to join the Christian uni groups. Attending our church makes me feel sick. And the longer I hide things, the harder it's gonna be to build the relationships back). It's gonna devastate them. I'm gonna have one hell of a summer.