The issue that Marcus Borg raises about Jesus being the way is an important one, but we need to be clear about the nub of the matter. Even most conservative Christians would not claim that there is no truth in other world religions, no knowledge of God to be gained from practicing other religions. The issue here is whether any of that knowledge or practice amounts to a saving knowledge of God, and the answer of various parts of the New Testament to that question is no.
For example, Acts 4:12 is very clear: "Salvation is found in no one else, for there is no other name under heaven given to human beings by which we must be saved" than the name of Jesus Christ. Or consider the words found in the latter part of the Pauline corpus: "For there is one God and one mediator between God and human beings, the man Christ Jesus, who gave himself as a ransom for all persons...." (1 Timothy 2:5). Thus, when we come to the text of John 14:6, it is important that we not read it in isolation from the rest of the things the early Christians were claiming they believed.
In some ways, the issue becomes clouded when we talk about religions, rather than talking about the person of Jesus Christ. None of the texts cited above are saying that the Christian religion, as a religion, saves. What is being claimed is that the person Jesus Christ is the one savior for the whole world. This is a rather different matter. Christianity has taken and does take many forms. But all those forms believe that the New Testament is the Word of God, and all of those forms know that there are a variety of verses in the New Testament canon that claim that Jesus is the only way of salvation and that faith in him is required if one is to be saved.
Thus it is right to be suspicious of Marcus Borg's all-too-modern interpretation of John 14:6. For one thing, the gospel of John is an example of an ancient biography, and the focus is on Jesus as a person--he is the vine, he is the good shepherd, he is the bread of life, and so on. To suggest that what "the way" means in John 14:6 is some sort of process of dying and coming to new life that may or may not involve Jesus is simply to misread the whole tenor and focus of this gospel. Jesus himself, the person of Jesus, and not some process of death and resurrection he went through, is said here to be the way to the Father. The meaning of this is much the same as 1 Timothy 2:5, for it is claiming that Jesus is the only mediator between us and the Father, being the only means of salvation.
Furthermore, Borg's attempt to drive a wedge between Jesus and a set of beliefs about Jesus will not do. The New Testament repeatedly says that Jesus is the savior, and faith in him is the subjective means by which one appropriates his benefits. The battle cry of early Christianity was "Believe in the Lord Jesus and be saved." Thus, salvation comes most certainly by the grace of God in Jesus, but it must be received through faith (Ephesians 1:13; 2:8), which involves a certain amount of understanding of and trust in basic truths about Jesus. Thus Christianity involves both the person of Jesus and various beliefs about him.
Finally, it is well to remember that, at the end of the day, if there is an all-knowing God who has set up the universe and made salvation available to us all in Christ, then it would be well to ask ourselves not "Why is God so set on doing this only one way?" but rather "Who are we to question the Eternal Being who knows far better than we do what is best for all of us? Who are we to say 'God, you can't do it that way?'" These are questions worth pondering for a long time, indeed. There is a potentially universal inclusivity in the exclusivity of claiming Christ as the only way of salvation. All can come to the Father, through Jesus the Son.