I wouldn't say Bahais are really a "biblical" religion. But the same could be said of other religions such as Islam, which accepts but at the same time re-interprets some biblical teachings, just as Mormonism does.
Jehovah's Witnesses have full confidence in the Bible as divinely inspired, and in no way do we believe that it is insufficient to acquire the knowledge needed to obtain eternal life. Jesus said:
John 17:3 This means everlasting life, their coming to know you, the only true God, and the one whom you sent, Jesus Christ.
If someone considers the Bible as a book truly inspired by God and Jesus Christ as the Messiah that the Creator had promised to his people in ancient times, by deduction they would accept that Jesus is the only means of salvation for men in consecuence of his own sacrifice. Only the knowledge that refers to Jehovah and his Son Jesus that he sent from heaven, are sufficient to obtain "everlasting life".
Actually, although there may have been many wise humans throughout the ages, not a single one of them was sent down from heaven by God, none of them died a sacrificial death to reconcile imperfect humanity to its Creator, none of them was declared His Son by the mouth of God Himself in front of other humans, none of them was resurrected and seen by a multitude of eyewitnesses as He ascended to heaven, none of them spoke from heaven to His disciples, and none of them was seated at the right hand of God as the future King of mankind... except Jesus.
That's what someone who accepts the Bible as divinely inspired believes, since those things are what it teaches.