Why does Christianity speak like this?
There are many voices in Christianity. Some see Christianity advancing until the whole world becomes Christian. This view while rather common, is not supported by the bible.
2Ti 3:1 This know also, that in the last days perilous times shall come.
2Ti 3:2 For men shall be lovers of their own selves, covetous, boasters, proud, blasphemers, disobedient to parents, unthankful, unholy,
2Ti 3:3 Without natural affection, trucebreakers, false accusers, incontinent, fierce, despisers of those that are good,
2Ti 3:4 Traitors, heady, highminded, lovers of pleasures more than lovers of God;
2Ti 3:5 Having a form of godliness, but denying the power thereof: from such turn away.
Christianity was always supposed to be about truth. However, it was not supposed to be a a system that was accepted because we understood and agreed with it. It was to resonate within the heart (those who seek truth).
Joh 18:37b Every one that is of the truth heareth my voice.
Many, often with good intentions, have come to repackage Christianity in different ways that exceed what is given us in the bible. As a result, some very distorted views of Christianity have been presented which can be confusing to Christians themselves much more to those looking at Christianity from the outside.
The difference between what can be fully understood and what can be trusted in was given voice by Peter;
Joh 6:66 From that time many of his disciples went back, and walked no more with him.
Joh 6:67 Then said Jesus unto the twelve, Will ye also go away?
Joh 6:68 Then Simon Peter answered him, Lord, to whom shall we go? thou hast the words of eternal life.
Joh 6:69 And we believe and are sure that thou art that Christ, the Son of the living God.