Why evangelize?

I am taking a course on the Biblical and Theological Foundation for Mission right now. For class I am reading Newbigin's book "The Open Secret." He has an interesting quote in one of the first chapters. I am sure I am no where near the intended point by not going further into his argument with you (he soon after begins to help you in responding to questions such as these - I am not satisfied with the answers). But I think these are questions worth asking - at least of myself:

Why not join with the sincere adherents of all religions in seeking the fullness of the truth to which they all aspire?

Why not join with all people of goodwill in tackling the real human problems of hunger, oppression, sickness, and alienation, instead of seeking more adherents for your religious group?

How would YOU respond to such questions?

Hindu to Christian

I am editing a friend's Masters Thesis for him. He is from Nepal but has come to the states for a couple of years to work on his ThM. The title is "An Analysis of the Idea of the Atonement in Hinduism in the Light of Christianity." It is actually quite fascinating. I came across this one section in his paper that I thought was especially probing...

"Does it really make much of a difference when a Nepali Hindu becomes a Christian? Both believe in doing good works, even in loving one’s neighbors; it’s just a question of motive. Hindus accumulate merit from their good deeds to counterbalance their bad deeds. Christians says that a life full of good deeds cannot outweigh even a single sin. We must first seek forgiveness from God relying on his mercy shown to us through his son, Jesus Christ. Then, once forgiven, he enables us to do good out of thankfulness and love for him who died for us."

To someone looking in from the outside - what's the difference? Why is the change necessary?