Surely if humans will ever make time machines, we would know about it?

This matter poses the theory, time machines could be built in the future.. but if we didnt know they changed the past, we would not know any different. They could of gone back in time and made up holy books, wars etc.

ps; If they have allready come back in time, then the future has allready happend for them? so they would be in the past present and future? Travel to the past, still in the present, but if they are in the past, then they came from the future?

just making conversation.