there really will be only one god in the world

Yes, they are 3 in 1. God the Father, God the Son and God the Holy Spirit. The logic is found in water, ice and steam. Colossians 2:9 "For in Him dwelleth all the fulness of the Godhead bodily."

only be one god for the world

My assessment of someone calling himself "Dark Poetry" who asks such agnostic questions is that he is sightseeing down the wrong path. You obviously have a need to prove God's existence before you acknowledge anything positive about Him. With this in mind, here's a quote from the late President Ronald Reagan that I've always found amusing and to-the-point: “I've had an unholy desire to invite some atheists to a dinner and then serve the most fabulous gourmet dinner that has ever been concocted and, after dinner, ask them if they believe there was a cook.” To answer your question, there is more than one God - man is just beholden to the one God who created him. The First Commandment states, "I am a jealous God - thou shalt have no other gods before me". If there is only one God, there would be no reason for Him to be jealous. It may be argued that He was talking about idols, and perhaps He was in this particular case. But going back to the creation of man, there is also an example where he states, "let us make man in our image". No idols existed then - "our image" is thus indicative of more than one God. Adam was not the first man on earth - there was already a civilization living here at the time. Adam was just the first man of our own God's family. This is why we are taught that there is only one God. "One God" doesn't mean there has only been one God that has ever existed - it just means that He is the one God that we will answer to and therefore, He is the one God that we are to be concerned with.