Saudi Arabia is an example of a state in which non-Muslim religion is effectively forbidden. However, it is the exception; Christians are tolerated in most predominantly Muslim countries, and indeed in some, such as Egypt and Syria, they are a large minority. Christians and Jews are regarded by Muslims as 'peoples of the book' who follow monotheistic religions that preceded Islam in terms of revelation.
It is true that Islam was spread by military conquest, and those following polytheistic or pagan beliefs had to convert or face death. However, Christians and Jews were tolerated, and many converted voluntarily to Islam.
One essential feature of Islam that attracted converts was its simplicity. For years, Christians had been embroiled in esoteric debates about the nature of Christ and the doctrine of the "Trinity", and many groups in the Middle East, such as the Copts and Nestorians, disagreed with the so-called 'orthodox' line on these issues, such as Christ being fully God and fully man, and simultaneously the Son of God and God the Son. Islam had a very simple doctrine - there is one God and Mohammad is his prophet. That was attractive to many who had tired of all the arcane disputes within Christianiry.
If you need a better insight into the differences between Christianity and Islam. Look under 'immigration' thread, under heading 'Islamisation of Britain' (last few pages)
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