Youre getting into questions of authority. The creator has delegated authority to certain other parties before certain choices were made which constitute treason against Him. Lucifer had authority as the primary archangel before inquity was found in him, and a war of rebellion ensued. He was not satisfied with being second under his Creator. After his removal from the presence of God, he was confined to this plane of existence which we now occupy.
So the question follows, if God is omniscient, and He knew of this rebellion before the first Word was ever spoken, why did he do it? Because He didnt want to be the only One. He found it good to create others like Himself.
Adam was the first of a new creation, a race of beings created in His very image.(The angels were not.) So, again, He gave authority over the earth, or this dimension, or this plane of existence, to Adam. Again, treason, so the authority fell by default to Lucifer, previously second in command under Adam. (Adam was specifically commisioned by God to guard against Lucifer.)
Jesus Christ showed up and restored authority back to God (which He is) and man (which He also is).
So now believers, under His authority, have the supreme authority in the earth. Unfortunately, we are born as natural beings, and we have to go through a process of learning to live as supernatural beings after our conversion. So the world is better, but still not perfect. But at least theres hope. So God is not the author of evil, and yes, Satan, or Lucifer, is indeed the god of this present age. But so are Christians, and there have been a number of us throughout history who really knew it and walked in it.
So, thats the big picture of theology. I didnt include scripture references for brevitys sake. (And my apostrophe key is broken, just so you spelling fascists know.)