GOD IS ETERNAL AND IMMORTAL

God is The One Being that He always has existed and always will exist.  He had no beginning and will never cease to exist. That is eternality. Those who has gained immortality among God's creatures are immortal but not eternal. They did never exist and never will  exist apart from God, and, on their own.

Note the technical difference between  eternalness and immortality: God is eternal because had He had no beginning.  He also is immortal because He will live forever. Humans with pure souls,  angels, demons, etc., are immortal because those will never cease to exist, but not eternal because they did not always exist.

It is important to realize that in practice eternal  and immortal are often used interchangeably and the technical difference is not always maintained.

When we say we are one with God that means we are living with Him and in His Power. We are never same as Him or we are Him.   God is separate from that which He has created.  The universe itself is not God but a reflection of His Glory.