Dear Anno :
First of all, the Bible says there is only one God.
“Unto thee it was shewed, that thou mightest know that the Lord he is God; there is none else beside him.”Deuteronomy 4:35 KJV
Secondly, we also know from the scriptures that the God of the Bible is absolutely perfect. For example, the Bible says,
“Be perfect, therefore, as your heavenly Father is perfect.
Deuteronomy 32:3-4 (NIV)
“I will proclaim the name of the Lord. Oh, praise the greatness of our God. He is the Rock! His work is perfect and all His ways are just; A faithful God who does no wrong, upright and just is he.”
Furthermore, there cannot be two absolutely perfect beings. Why? Because, if there were two absolutely perfect beings, they would have to be different in some way; otherwise, they would be the same being. In order to be different, one of the beings would have to have an attribute the other did not have. That means the other being would lack that attribute, and therefore would not be absolutely perfect. Hence, there can be only one absolutely perfect being.
Finally, there seems to be only one Mind behind the whole universe who was doing the pre-planning of the whole thing; there is only one set of physical laws in the whole universe which reflects one Mind behind it all. This is the main reason it is called a uni-verse (one world from one Mind) not a multi-verse (many worlds from many minds).
For the above reasons, we say that there is only one true perfect God
The Bible further supports our position as follows:
“Know therefore this day, and consider it in thine heart, that the Lord he is God in heaven above, and upon the earth beneath: there is none else.” Deuteronomy 4:39 KJV
“As concerning therefore the eating of those things that are offered in sacrifice unto idols, we know that an idol is nothing in the world, and that there is none other God but one.” 1 Corinthians 8:4 KJV
“There is one body, and one Spirit, even as ye are called in one hope of your calling; One Lord, one faith, one baptism, One God and Father of all, who is above all, and through all, and in you all.” Ephesians 4:4-6 KJV
“Is he the God of the Jews only? is he not also of the Gentiles?
Yes, of the Gentiles also: Seeing it is one God, which shall justify the circumcision by faith, and uncircumcision through faith.”
Romans 3:29-30 KJV
“For there is one God, and one mediator between God and men, the man Christ Jesus;” 1 Timothy 2:5 KJV
“yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live.” 1 Corinthians 8:6 NIV