Key Verses: John 14:6 Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me.
Acts 16:30 And brought them out, and said, Sirs, what must I do to be saved? 31 And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house.
The question implies first of all that there is a need to be saved, but saved from what? From sin, which in its most basic definition is anything we do, or fail to do, that is not pleasing to God. Sin is the universal human condition; we all inherited our sin nature from Adam (Romans 5:12), and because of sin, we are all destined for death (Romans 6:23). If we are saved, then, we are saved for life, not just in the life we live here, but everlasting life (John 3:16). Those in the world who concede the need to be saved from our depravity don't dispute the ends, but the means. The Christian insistence on Jesus Christ as the only way to God is too narrow and restrictive, they say, deliberately overlooking the fact that Jesus himself described the Way as narrow (Matthew 7:14). It is the height of presumption for the creature to question the Creator's wisdom.
The reason for the insistence on there being more than one way to God is human pride. We want to trust in our own wisdom, and go about doing as we please in the blithe assurance we'll obtain an everlasting reward anyway. If it were possible for any human being to please God through his or her own efforts, then Christ need not have died on the cross. Any other way than the way of faith in Christ is salvation by works, and doomed to failure (2 Timothy 1:9). Satan has counterfeits for everything Christ offers; for the Bible, he substitutes the Koran, or the Book of Mormon; for faith, he substitutes a gospel of works, the same gospel the Jews tried to force on the early church in the guise of the law (see especially the epistle to the Galatians). In these latter days, he grows ever more subtle; thanks to the myriad of scripture translations, those who espouse almost any belief can point proudly to a scripture translation that appears to endorse their views.
People can't be saved by some emotional frenzy, or by an appeal to intellectual pride. They can't come to the Father through Mohammed, or Buddha, or Joseph Smith. If we accept the Bible as God's authority, and insist on its traditional interpretation, we cannot justify any other way to be saved than through Jesus Christ. Those who contend otherwise are not of God, and the Bible says clearly that anyone who tries to preach another gospel besides that of salvation through Christ is accused (Galatians 1:8-9). God's curse doesn't mean much to someone who doesn't even believe he is, but God will not forget. Those who reject Christ are damned for eternity; those who teach others to reject him are doubly damned, because the blood of those they have misled is on their own heads (Ezekiel 33:6).
The Holy Spirit is God's guarantee that we are his, and that his promises are true (2 Corinthians 1:22). If we don't have that testimony within us that we are God's, then we have never been saved. Assurance, the certainty of our salvation, is a gift God gives to all of his children. If we are counting on being "good enough" to get into Heaven, we'll never live there, because without faith it is impossible to please God (Hebrews 11:6). There is none other in whom we can have that faith than in Jesus Christ.