For me, and for others I've discussed this with, the issue is with the binary nature of salvation.

The majority of people look in it as being "Accept Christianity and go to Heaven, reject Christianity and go to Hell." The inference drawn from this is that a child-molesting serial-killing rapist has only to accept Christ to have all his heinous acts nullified and stroll straight into Heaven. In the meantime the Hindu/Muslim/Sikh/Buddhist/Atheist who has spent their entire life in the selfless service of others is damned to the Lake of Fire.

Hardly just and equitable, is it?

Under many of the old faiths, of course, people were judged after death on their deeds, rather than their beliefs. Thus in Hellenism, three judges waited at the entrance to the Underworld: Aecus to judge Europeans, Rhadamanthys to judge Asiatics and Minos for the difficult cases. It was their decision, based on the persons' actions in life, that decided the souls' destination, Elysium, Asphodel or Tartarus. Didn't matter which Gods you'd followed in life, either, since even those who hadn't heard of the Gods would be judged in the same way.

Much the same went for the Norse. Die in battle, get picked up by a mezzo-soprano on a winged horse and go to Valhalla. (I have no doubt that a Norseman, had he heard of them, would expect to meet Leonidas and his 300 in Valhalla!). Those who died in other ways went to Hel, a gloomy place beyond Nifelheim, where the Queen, Hel Lokisdottir, would expect you to live and work as you had done in life, except in her service. Traitors, murderers and the dishonoured were sent by Hel to Nastrond, a bleak coast, where they were prey for the Nidhogg who sucked their blood and packs of wolves who tore them, but they healed overnight so they could be tormented more the next day.

But again, this was to do with your deeds in life, not the faith you professed. Islam and Christianity both share the same flaw. Judgement based on. often peri-mortem, profession of faith as opposed to the totality of your actions in life.

It puts people off because it's unfair.