Contrary to Sunnis- who claim Allah ﷻ is unlike creation, and thus incorporeal- the Mujasima believe that God is a body.
Body: a being with dimensions (a height, width, or depth) stretched out in space.
We argue for God’s incorporeality with the following syllogism:
- Allah did not begin to exist.
- All bodies began to exist.
- Therefore, Allah is not a body.
Premise 1: Allah did not begin to exist
The truth of this first premise is agreed upon between the Sunnis and the Mujasima.
Premise 2: All bodies began to exist
The second premise is true, because it is impossible for a beginningless body to exist. This is because a body is inseparable from either the state of motion, or the state of rest. So if a beginningless body existed, it would either have been:
- Moving since eternity past
- Resting since eternity past
- Alternating between motion and rest since eternity past
All three of the above is impossible, and that which entails impossibility is impossible. So it is impossible for a beginningless body to exist. Proof for the impossibility of each category follows below.
Moving Since Eternity Past
This is impossible because the state of motion exists contingently for a body. Meaning: a body that is moving, accepts rest. Since the state of motion exists contingently, this means it was brought into existence by some specifier that selected motion over the possible alternative. And since the state of motion was brought into existence, it could not have been beginningless. Therefore a body moving since eternity past is impossible.
Resting Since Eternity Past
This is impossible for the same reason the first category was impossible. The state of rest exists contingently. A body that is at rest, accepts motion. Accordingly, the state of rest could not have been beginningless.
Alternating Between Motion and Rest Since Eternity Past
This is impossible because each alternation from one state to another is an event in time, and it is impossible for an infinite number of events to have been completed in the past.
Therefore, Allah is not a body
The two premises are true, so the conclusion necessarily follows. This means that it is impossible for Allah ﷻ, who is beginningless by agreement, to be a body.
 Mujasim: a person who believes that God is a body. This belief is called Tajsim.
 Given the impossibility of Tasasul.
The occurrence of an infinite number of past events is impossible, because the past is that sequence of events which leads up to, and then ends with the present moment. To claim that the past is infinite, is therefore tantamount to claiming that a sequence which came to an end is endless. And that is a clear contradiction. More on this here.