Christians differ on this point. There are three main theological positions:
1) EXCLUSIVISTS: Most evangelicals and conservative Protestants believe that faith in Jesus is the only path to salvation, and those who do not have a "relationship" with Jesus, through being born again, are eternally lost.
2) INCLUSIVISTS: Roman Catholics and moderate Protestants believe that the grace of Jesus, mediated through the Church, is the only means of salvation, but that this grace is available to those outside the Church, and may be mediated indirectly through other religions such as Islam, Judaism, etc.
3) PLURALISTS: Liberal Catholics and Protestants believe that all religions are paths to God, and there is salvation for all Muslims, Jews, etc.
Personally, I am a moderate Inclusivist - I believe that there is salvation for those outside the Church, but all salvation is mediated through the Church. Other religions may approach God, and their belief systems may have divine inspiration, but they lack the core of salvific grace which is in the Church. This is quite an ancient Christian belief, and is the official position of the Roman Catholic Church after Vatican II (although I am not a Roman Catholic).