If there are groups that believe God is a substance, where is such a belief based? When did this belief started?
The one God is substance itself and form itself. Angels and people are substances and forms from him. To the extent that they are in him and he is in them, to that extent they are images and likenesses of him. Because God is the underlying reality, he is also substance. Unless the underlying reality becomes substance it is a figment of the imagination; but as a substance it becomes an entity. And one who is substance is also form, for substance without form is another figment of the imagination. We can attribute both of these to God, provided he is seen as the sole, the only, and the archetypal substance and form. (True Christianity #20)
God is substance itself and form itself, and is therefore the first and only substance and form, whose essence is love and wisdom. All things that were made, were made by God [John 1:3]. It follows, therefore, that it was from love by means of wisdom that God created the universe and each and every thing in it. As a result, divine love together with divine wisdom is present in every single entity that has been created. Furthermore, love is the essence that not only forms all things but also bonds and unites them to each other; therefore love is the force that holds all things in connection. (True Christianity #37)
Everyone who thinks rationally and clearly also sees that everything has been created out of a substance that is substance in and of itself. This is the essential being from which everything that exists can arise. Since only God is substance in and of itself and is therefore essential being, it follows that there is no other source of the arising of things. (Divine Love and Wisdom #283)
The substance of God, according to Swedenborg, is love:
This makes it clear that Divinity, being substance in its own right or the unique and sole substance, is the source of absolutely everything that has been created. This means that God is the sum and substance of the universe, in accord with what was presented in part 1: divine love and wisdom is substance and form (40-43); divine love and wisdom is substance and form in its own right, and is therefore wholly itself and unique (44-46); everything in the universe was created by divine love and wisdom (54-60 [52-60]); the created universe is therefore an image of him (61-65 [61-64]); and [in part 2,] the Lord alone is the heaven where angels live (113-118). (Divine Love and Wisdom #198)
And from the first of the sections Swedenborg refers back to here:
Divine love and wisdom is substance and is form. The everyday concept of love and wisdom is that they are something floating around in, or breathed out by, thin air or ether. Hardly anyone considers that in reality and in function they are substance and form.
Even people who do see that love and wisdom are substance and form sense them as something outside their subject, flowing from it; and they refer to what in their perceptions is outside the subject and flowing from it as substance and form even though they sense it as floating around. They do not realize that love and wisdom are the actual subject, and that what they sense as floating out from the subject is only the appearance of the inherent state of the subject. (Divine Love and Wisdom #40)
Divine Love and Wisdom was first published in Latin in 1763. True Christianity, traditionally titled True Christian Religion, was first published in Latin in 1771.
In short, Swedenborgian Christians believe that God is substance, and that that substance is love. This is in accordance with the Apostle John's statement:
Whoever does not love does not know God, for God is love. . . . God is love, and those who abide in love abide in God, and God abides in them. (1 John 4:8, 16, emphasis added)
Are there Christians that believe that God is a substance?
St. Thomas Aquinas believed that God is substance: Divine Substance.
The Catholic Church teaches it as thus:
In the Aristotelian thought of St. Thomas Aquinas, essence is synonymous with nature and substance. Therefore this teaching holds that God’s nature (his Godhead) is exactly who he is: there is nothing added to the substance of God. Whereas all living things have a substance (the primary mode of being) and accidents (the secondary modes of being), God is only a substance.
This teaching may be confusing to those who are unfamiliar with the philosophic terminology of Aristotle and Aquinas. Essentially, Aristotle held that all created beings are defined not by their constituent parts (such as atoms) but by substance. Substance is the nature of the being. Additionally, all created beings have accidents which inhere in the substance. An accident is a secondary mode of being which involves change. Typical accidental modes of being are quantity, quality (such as color, texture etc), action, posture, time, state and relation to something else. In this sense we can say that a man is composed of a substance (humanity) and a list of accidents which make him a unique person (such as height, weight, color, position, time of existence, etc.). In this light, we can see that the phrase, "God is the same as his essence" implies that God has no accidental modes of being and is defined only by his substance (his Godhead).
Definition of the Dogma:
This teaching is not a defined dogma of Catholicism, but rather a speculative theological teaching of St. Thomas Aquinas. However, since this teaching is drawn from the dogma of God’s simplicity is at least a Sententia Communis doctrine. - God is the Same as his Essence
Here is how St. Thomas in his Summa Theologiae (The simplicity of God) puts this metaphysical subject:
Whether God is the same as His essence or nature?
God is the same as His essence or nature. To understand this, it must be noted that in things composed of matter and form, the nature or essence must differ from the "suppositum," because the essence or nature connotes only what is included in the definition of the species; as, humanity connotes all that is included in the definition of man, for it is by this that man is man, and it is this that humanity signifies, that, namely, whereby man is man. Now individual matter, with all the individualizing accidents, is not included in the definition of the species. For this particular flesh, these bones, this blackness or whiteness, etc., are not included in the definition of a man. Therefore this flesh, these bones, and the accidental qualities distinguishing this particular matter, are not included in humanity; and yet they are included in the thing which is man. Hence the thing which is a man has something more in it than has humanity. Consequently humanity and a man are not wholly identical; but humanity is taken to mean the formal part of a man, because the principles whereby a thing is defined are regarded as the formal constituent in regard to the individualizing matter. On the other hand, in things not composed of matter and form, in which individualization is not due to individual matter—that is to say, to "this" matter—the very forms being individualized of themselves—it is necessary the forms themselves should be subsisting "supposita." Therefore "suppositum" and nature in them are identified. Since God then is not composed of matter and form, He must be His own Godhead, His own Life, and whatever else is thus predicated of Him.
God is not only His own essence, as shown in the preceding article, but also His own existence. This may be shown in several ways.
First, whatever a thing has besides its essence must be caused either by the constituent principles of that essence (like a property that necessarily accompanies the species—as the faculty of laughing is proper to a man—and is caused by the constituent principles of the species), or by some exterior agent—as heat is caused in water by fire. Therefore, if the existence of a thing differs from its essence, this existence must be caused either by some exterior agent or by its essential principles. Now it is impossible for a thing's existence to be caused by its essential constituent principles, for nothing can be the sufficient cause of its own existence, if its existence is caused. Therefore that thing, whose existence differs from its essence, must have its existence caused by another. But this cannot be true of God; because we call God the first efficient cause. Therefore it is impossible that in God His existence should differ from His essence.
Secondly, existence is that which makes every form or nature actual; for goodness and humanity are spoken of as actual, only because they are spoken of as existing. Therefore existence must be compared to essence, if the latter is a distinct reality, as actuality to potentiality. Therefore, since in God there is no potentiality, as shown above (Article 1), it follows that in Him essence does not differ from existence. Therefore His essence is His existence.
Thirdly, because, just as that which has fire, but is not itself fire, is on fire by participation; so that which has existence but is not existence, is a being by participation. But God is His own essence, as shown above (Article 3) if, therefore, He is not His own existence He will be not essential, but participated being. He will not therefore be the first being—which is absurd. Therefore God is His own existence, and not merely His own essence.
This depends on what you mean by substance. If you mean physical material that can be touched or seen then God is the all-pervading spark of creation and destruction in all its material and immaterial forms. Perhaps you mean substance as in something that exists? Then yes, from a trinitarian and non-trinitarian view God is THE substance. The final question and answer.
It's a difficult question because your trying to define that which is indefinable but from my own research the closest thing I have found to answer your question is Spinozism. This is Baruch Spinoza's philosophy of god that delves quite deep into the idea of god and substance. His definition of substance as it pertains to God is that "By substance I understand what is in itself and is conceived through itself, i.e., that whose concept does not require the concept of another thing, from which it must be formed. Basically that substance is whatever that can be thought of without relating it to any other idea or thing.
I believe that of course God is a material substance but he is also everything that is not substance. The Father is life and death, creation and destruction, the matter and anti-matter. There is nothing you can imagine that isn't of the Almighty. Of course there are sects of Christianity that believe all types of different ideals and dogma far surpassing the idea of God as physical form, but pinpointing the exact creation of said offshoots is near impossible.