The term “Sun of God” was introduced by Jesus Christ through B.Cullen — a prophet of the past century, who lived in the USA. The book was published in the USA in 1992 and then republished in Russia  (unfortunately with many spelling errors). The language and the content of the book correspond to the world outlook of the person who received the information: God always talks to people using the language and the terms they can understand. (It is due to this that the book describes pictures of a spaceship and Christ leaving the Earth on it). But the book also contains very valuable information.
The main message that Jesus wanted to get across to people through this book is the following.
God is always a Living God. Jesus was on Earth 2000 years ago, and He exists today among embodied people (although He is not embodied now). But many believers guided by ignorant pastors are engrossed in an absolutely unpromising and meaningless exercise: performing endless ritual bodily movements, repeating prayers that sometimes are not only useless, but also harmful. They waste their time, instead of making efforts on a real spiritual self-transformation through realization of the God's program for them — that consists in cognition of God and merging by consciousness with Him in Love. Jesus says that He walks in churches among the believers who appeal to Him in their prayers — yet they do not see Him, do not feel Him, do not listen to, and do not hear Him...
Instead of doing this silly exercise, Jesus suggests that His followers serve God by serving people, as well as make personal efforts on cognizing the Creator. It is also foolish to seek God in heaven: one should seek the spiritual “Heavens” not above the surface of our round planet, but in the deep of the multidimensional space. And the Abode of the Creator is very close to us: the distance to it is smaller than the thickness of a paper. And the way to the Abode begins inside one’s developed spiritual heart.
The Creator (God-the-Father) sometimes names Himself the Sun of God  — of God in the aspect of the Absolute [4-6]. That is, He — the Creator Who is in His Abode — is the principal part of the multidimensional Absolute. The Absolute fills entire universal space.
The entrance into the Abode of the Creator can be seen by successful spiritual seekers, who approached Him, as a giant and tender Sun of God that resembles a caressing morning sun, only thousands times bigger. To cognize all this and to enter through the Sun of God into the Heart of God is the ultimate purpose of our life, of the personal evolution of each and every one of us (see also [2,3,6-7]).
On the Path of our personal self-perfecting and in our efforts to help other spiritual seekers, the Holy Spirit is always willing to help us. Jesus describes It as Power of unlimited capabilities that comes out from the Abode of the Creator. The Holy Spirit is always willing to help worthy spiritual seekers, always awaiting the situations to do it.
But we should make our state close enough to the state of the Holy Spirit and Jesus Christ to see, feel, and hear Them, to merge with Them in Love and become like Them and then to cognize the Creator with Their help.