The unimaginable God is beyond vision [invisible] and beyond imagination [unimaginable]. Some entities are imaginable but invisible like cosmic energy. The cosmic energy was also unimaginable for some time, when the science was not developed. The cosmic energy was temporarily unimaginable and this does not mean that there is no permanently unimaginable.
The roof of the house is temporarily untouchable, but after some practice of high jump, it can be touched. The cloud is permanently untouchable because you can never touch the cloud by high jump in any time in the future. Hence, the cloud is permanently untouchable. You may argue that by taking an aero plane even the cloud be touched. Now you may argue that there is no permanently untouchable item in view of the advanced technology of science.
You can never touch the boundary of the universe with the help of any technology of science in any time in future. This is permanently untouchable item. You can never reach the boundary of this universe. Imagining a compound wall is assumed. If such compound wall exists, what is present beyond the compound wall? Therefore, the boundary of universe is always unimaginable and stands for the unimaginable God. This means that God is beyond the creation.
Islam considers a wall as the representative of God. The significance is the wall stands for the boundary of this creation which is unimaginable God. Therefore, you can never see or even imagine the original God. The unimaginable events performed by God, which are called as miracles also indicate the unimaginable God only. But, even these miracles require imaginable and visible media to establish the concept of unimaginable nature. A boy called Krishna lifted the huge mountain with His tender finger.
The boy, the mountain, the tender finger and the process of lifting are imaginable and visible items only. All these established the existence of unimaginable nature of tender boy lifting the mountain. This unimaginable event is a proof for the existence the very unimaginable nature called as God. The performer of this unimaginable event must be the possessor of God. The human form alone can preach the knowledge and clear all the doubts. The human form is the complete representation of all the items of the nature, which are the five elements, life, mind and intelligence. Therefore, inert items which are composed of five elements only cannot be the complete medium.
Even the living beings like birds and animals having life and mind are also not the complete medium. Some birds and animals possess intelligence to some extent. The human form alone is the complete medium possessing all these items of nature.
It also has the capability of expression through perfect language for preaching the knowledge to human beings. Hence, human form alone is the address of the medium of unimaginable God for the sake of human beings. Bhagavatam explains the importance of such human form of the Lord and therefore Shuka was the preacher of human form of unimaginable God.