Here's a midrash that's quoted fairly often - and usually in support of a rather odd theological position. The truth is, as you will soon see, that a simple reading of the midrash does seem to support this position. Does this mean that we must now reconsider a fundamental belief of Judaism?
בראשית רבה פרשה סח ט
ויפגע במקום ר' הונא בשם ר' אמי אמר: מפני מה מכנין שמו של הקב"ה וקוראין אותו מקום? שהוא מקומו של עולם, ואין עולמו מקומו, מן מה דכתיב (שמות לג): הנה מקום אתי, הוי, הקדוש ברוך הוא מקומו של עולם, ואין עולמו מקומו.
אמר רבי יצחק: כתיב (דברים לג): מעונה אלהי קדם, אין אנו יודעים אם הקב"ה מעונו של עולמו ואם עולמו מעונו, מן מה דכתיב (תהלים צ): ה' מעון אתה, הוי. הקדוש ברוך הוא מעונו של עולמו, ואין עולמו מעונו.
אמר רבי אבא בר יודן: לגבור, שהוא רוכב על הסוס וכליו משופעים אילך ואילך, הסוס טפילה לרוכב ואין הרוכב טפילה לסוס, שנאמר (חבקוק ג): כי תרכב על סוסך.
Beraishis Rabba 68:9 And he met up with the place (Gen. 28:11) - Rav Huna said in the name of Rabbi Ami: why do we use "place" (מקום) as a nickname for God? Because He is the place of the world, and His world is not His place. (This we know) from the verse (Ex. 33:21) "behold there is a place with Me..." Thus, the Holy One, blessed be He, is the place of the world and His world is not His place.
Rabbi Yitzchak said: It is written (Deut. 33:27) "The Eternal God is a dwelling..." But we don't know if the Holy One, blessed be He, is the dwelling of His world or if His world is His dwelling. (We can deduce this) from the verse (Psalms 90:1) "Lord, You are a refuge..." that the Holy One, blessed be He, is the dwelling of His world and His world is not His dwelling.
Rabbi Abba bar Yudan said: (this can be compared to) a warrior who is riding on a horse, his weapons hanging from this side and that. The horse is dependent on the rider, but the rider is not dependent on the horse, as it says (Chabakuk 3:8): "You ride on Your horse..."
Now some have inferred from this midrash that God is the "place of the world" in the sense that the physical universe is somehow "contained within Him."
The problem is that this formulation directly contradicts at least two of Rambam's thirteen principles!
Here's Rambam's second principle:
The Unity of God, may He be blessed. That is to say that we believe that the Cause of everything (i.e., God) is One. But not like "one of a pair" or "one of a class" or like one man who is made up of many parts or like a single simple body that can (still) be divided...Rather, God, may He be blessed, is One in a way that has no comparison...
...Meaning that God is infinitely simple and, thus, cannot be divided - he has, in other words, no parts.
And here's Rambam's third principle:
The absence in Him of any physical quality. That is that we believe that this One (God) that we have mentioned has no body nor any physical power nor do any physical events affect Him...
...Meaning that, however you choose to think of God, there can be no physical quality of any kind in the equation. To say, therefore, that the physical universe is somehow "part" of Him, is to reject these two of the Rambam's principles.
But still, we can't just ignore the words of the midrash! If we can't find a viable and equally straightforward way to read it, then we're left in a rather uncomfortable position.
Fortunately Rambam himself discussed this very midrash in More Nevuchim (1:70). He observes that the rabbis (Chagiga 12b) characterize God as dwelling "above the heavens" (adding the word עליהם) and not "in the heavens" (שוכן בערבות) - which would incorrectly imply that God is somehow a part of the physical universe, taking up space. Thus, God's relationship to the universe is like that of a master who controls from a distance, but He is not a participant.
Rambam then quotes our midrash, and comments:
And you should know...that a rider is greater than what he is riding. We do not mean that literally, but we use this as a figure of speech, for a rider (can't be compared in any way to what he is riding) because they are of fundamentally different categories. And furthermore, since the rider can restrain the animal (he is riding) and guide it according to his will, it is a tool in his hand...
So too with God, His Name should be exalted, He restrains (i.e., guides) the upper sphere...and (yet) He is distinct from it...
In other words, the analogy in the midrash to a rider on his horse is actually meant to explain the first two paragraphs ("place" and "dwelling"): just as a rider controls his horse, but can in no way be confused with it, so God manipulates and guides the world but must never be confused with his creation.
With this "new" reading, I believe we can confidently consider the theological principle of God's Unity and the midrash reconciled.
I should note that the "Ani Maamin" poem found in some siddurim has very little in common with the Rambam's actual principles and has no theological authority.