Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead.

Because he has chosen a specific day for judging the world fairly, through the person he selected. He has proven this to everyone by bringing him back to life after he died.

King James Version

Simple English

* Work in progress - there may be occasional inaccuracies