Well, as I told Mawia, precision of words is necessary here. We can say "form" in English, but what do we mean in Greek? I suggest that God does have a form, and by "form," I mean the Greek word εἶδος (eidos).
In John 5:37, it is written,
καὶ ὁ πέμψας με πατὴρ αὐτὸς μεμαρτύρηκεν περὶ ἐμοῦ οὔτε φωνὴν αὐτοῦ ἀκηκόατε πώποτε οὔτε εἶδος αὐτοῦ ἑωράκατε
which is translated as,
and the Father who sent me, the same testified about me. You have neither heard His voice nor have you seen His εἶδος.
Firstly, εἶδος can have several meanings in Koine Greek, but here, it's evident that εἶδος is something that can be seen, as indicated by the verb ἑωράκατε ("have seen"). BDAG primarily defines it as "the shape and structure of someth[ing] as it appears to someone, form, outward appearance," even citing John 5:37 as an example.
Jesus told the Jews, "You have neither heard His voice..." --- referring to the voice of God the Father. While we must certainly believe that those Jews (to whom Jesus was speaking) had never heard God the Father's voice --- since Jesus said so --- are we to assume that God the Father does not have a voice that can be heard? Clearly NOT, for whose voice but God the Father's declared, "This is My beloved Son in whom I am well-pleased" (Matt. 3:17 cp. Luke 3:22; Mark 1:11)? So, while those Jews had not heard the voice of God the Father, they potentially could have. Meaning, God the Father does have a voice capable of being heard.
Now, that being said, Jesus also told those Jews, "...nor have you seen His εἶδος." Again, although the Jews had not seen the εἶδος (that is, the form) of God the Father, are we to assume that God the Father does not have a "form" (εἶδος)? Clearly NOT. The structure of the verse does not allow it. Those particular Jews had not heard God the Father's voice, but God the Father has a voice to be heard. Otherwise, Jesus' reproach is hollow and nonsensical. Likewise, those particular Jews had not seen God's the Father's form (εἶδος), but God the Father has a form (εἶδος) to be seen! Is this not what the verse clearly implies?
The real question is, then, not whether God has a εἶδος, a "form," but rather:
- How can it be seen?
- How does it appear to the eyes?
- Must having an εἶδος imply corporality? (This many assume, but why must this be so?)
In addition, it is most certainly true that the persons of the Holy Trinity (with respect to the Son, His pre-incarnate existence) are invisible (ἀόρατος) (Col. 1:15). But isn't Moshe described as "seeing Him who is invisible" (Heb. 11:27)? Did not Jesus also say that the angels in heaven behold the presence of God the Father (Matt. 18:10)? Clearly, Moshe saw something. Clearly the angels see something. They may not see it as we do, and we may not be able to see what they have seen (not now at least), but nevertheless, is there not something to be seen? Jesus said (Matt. 5:8), "Blessed are the pure in heart, for they shall see God." Can something be seen unless it has a form or appearance (εἶδος)?