For Christians at least the answer is one, very simple, phrase: "The Great Commission".
And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the world. Amen. (Matthew 28 18-20)

And he said unto them, Go ye into all the world, and preach the gospel to every creature. He that believeth and is baptized shall be saved; but he that believeth not shall be damned. And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover. So then after the Lord had spoken unto them, he was received up into heaven, and sat on the right hand of God. And they went forth, and preached every where, the Lord working with them, and confirming the word with signs following. Amen. (Mark 16 15-20)
In addition to The Great Commission, Christians will also often refer to 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.
The Great Commission, as it has been known for most of this millennium has been used to justify Christian projects from The Crusades to present day Televangelism. It is the primary basis for all Christian evangelism.

Christians would hear "You don't need others to believe what you do, you need to believe it yourself" and respond, "I do believe therefore, in order for you to be saved, you must believe too."
Note that I do not necessarily agree with this opinion; I simply state fact.