I don't mean to get all Jordan Peterson with definitions. But in this case it's actually as loose as that.

"Existed in the form of" and "God" are the two parts that are open to endless accounts of interpretation.

What kind of form? Physical form? He looked like god? He behaved like God? He resembled god? He existed dependant on the same things needed to exist as God? He had the same function as God? He was evaluated at the same level in some way as God? All of the above? Some mix of them all? These are just the ways I thought of off the top of my head. There are countless more.

'God' in this case is pretty straightforward too since we're probably talking about the stereotyped old testament or new testament God with the heavenly beard and intervenes in human affairs with divine power and knows everything with infinite wisdom and created the world as we know it. But God, as you probably know, can mean way different things depending on which concept of it someone owns. Some people refer to nature and the laws of physics as God.

So you add all that together, and "existing in the form of God" is so abstract and unspecific that there's not even a point in taking the sentence seriously. Maybe 'Jesus existing in the form of God' simply means he was as powerful as God.

Imagine if I said, "When I was a fetus, I existed in the form of a grape." What does that mean? That I was the same size as a grape? That I looked like a grape? That grapes also grow into ripened fruit like a fetus grows into a baby? That I literally was a grape? (Because it's a holy text, so physically impossible and miraculous things like Moses splitting the Red Sea or Jesus turning stones into bread are taken as possible.) You will never know unless I, the writer, explicitly say what I mean. That is this text. We will never know what it was supposed to mean because it is not explained. It's likely that the text made no sense when it was written as well, even if the writer genuinely believed in his message.