You asked
-how could (you) differentiate if some old book contains the word of god or if it is just another book written by humans?-

So I sent you a song that shows people that they already do 'serve somebody'

I was polietly saying to you that you have already made a decision, so what you should have asked is '-how do I know if the god that I am currently serving is the true god, or do you (me) think that you should start believing writtings that tell us an incredible story about GOD-"

To the question that you should have asked I would say that the Roman goverment thought that the GOD (of thoes writtings) was such a credible threat that they crusified GOD, and then they sealed the tomb and posted soliders at the tomb.