Eternal sin
Thomas Aquinas lists, or has responded to, six sins that supposedly go against the Holy Spirit:
Thomas Aquinas explains that the unforgivability of blasphemy against the Holy Spirit means that it removes the entrance to these means of salvation; however, it cannot hinder God in taking away this obstacle by way of a miracle.
Christianity
With around 2.4 billion adherents, split into three main branches of Catholic, Protestant and Eastern Orthodox, Christianity is the world's largest religion. The Christian share of the world's population has stood at around 33% for the last hundred years, which means that one in three persons on earth are Christians. This masks a major shift in the demographics of Christianity; large increases in the developing world have been accompanied by substantial declines in the developed world, mainly in Europe and North America. According to a 2015 Pew Research Center study, within the next four decades, Christians will remain the world's largest religion; and by 2050, the Christian population is expected to exceed 3 billion.