As you know, I am a Christian; what some people don't realize that my family on my maternal grandmother's side were Jewish. I told this to a Rabbi, who informed me I was considered a Jew. The trouble is that I have never practiced Judaism at all. My grandmother married a man who was born in Italy and was raised a Catholic and they raised their children, including my mother, as Catholics. My mother, however, abandoned all religion before I was born; so I was mainly raised with no religion.
My question is this: Can I consider myself a Messianic Jew although I was never a practicing Jew? I'll understand if you say that I can't, I just wanted to ask.
Shalom!
A common question, even more common in South America, where half of my family is from. Just about everyone with European background (as opposed to Native) is of at least part Jewish descent.
First, let me point out the sticky (a work in progress) over in the Jewish DIR for reference:
http://www.religiousforums.com/forum/judaism-dir/167758-future-sticky-who-jew.html
The Rabbi you asked probably was Conservative or Orthodox, as technically since your mother's mother was Jewish that made your mother Jewish therefore you are as well.
But as you know, it's not that simple. I served several years as Outreach chair in my Reform shul, and in a case like yours (and each case is unique) I'd follow the thought in Reform that some Jewish identity must have been observed, however simple. Since in your case and in very many cases that is zero, I would recommend the conversion process be followed, as if no Jewish background was present. Because if no Jewish rituals or customs were in a persons life, well, it's just not fair to expect them to take on the whole of the faith's duties and responsibilities without some help. It's considered a kindness to say to the person with Jewish blood but no Jewish upbringing: 'Let's take things slowly'. The Reform 'Intro to Judaism' course (the first step in the conversion process) is very good indeed, I've taken it just to see for myself.
So, on to your question (sorry it took so long) of if you can consider yourself a Messianic Jew... ask yourself if you consider yourself a Jew first. Yes, set aside the Messianic part, for now. Ask yourself honestly if you identify as a Jew? If the answer is unclear, then ask yourself if you wish to? This is not a trivial question, you have a choice to make. If the answer is yes, then you need to 'get up to speed' so to speak in Judaism. Approaching a Reform Rabbi for conversion advice could be a start. (again, setting aside Messianic for now) Other branches, they would not require conversion process but the need for Jewish education would still be there in full, because you'd be quite lost without it.
Ultimately, to consider yourself a Messianic Jew, you'd have to consider yourself a Jew first.
Hope this helps!