QUESTION: "I know that ultra-Orthodox Jews do not think of converts as fully Jewish, although I did go to mikvah as well as do all the other things for my conversion."
ANSWER: Where could you have possibly gotten this idea from??? I'm shocked to hear someone is saying such things. If you had a conversion according to Jewish law, then everyone accepts that you are 100% Jewish. If your conversion was not according to halacha, then every possible type of Orthodox Jew doesn't believe that you are even partially Jewish.
The problem in your case may be that if you didn't have a rabbi that believes in the absolute binding nature of Torah law upon every Jew, then he cannot bind you to the Torah either. That is part of Jewish law, and at least as important as mikvah.
QUESTION: "How can you know for certain if you are supposed to convert? Is it a sin to delay the conversion if your heart desires it and you are constantly without peace of mind?"
ANSWER: CLEARLY there is no sin involved, especially since you seem to still have lingering doubts about whether conversion is the correct step for you. Have you spoken about it with a rabbi who is qualified to answer the process?
QUESTION: "Shalom! I am a 43 year old male with Jewish wife and 2 Jewish children. I feel very strong for the Jewish people and I like very much to read the Torah. We have thought of maybe moving to Israel in the future and I feel that I want to be a part of the Jewish people. I am born Christian, and I wonder if and how much it takes to convert to Judaism, or if I can live as a Christian with Jewish values?"
ANSWER: My personal opinion is that for yourself, as an individual, there is no necessity to convert if you don't feel a strong inner desire to do so. You can, as you say, "Live as a Christian with Jewish values," and the Al‑mighty will be pleased with you.
However, as the father of Jewish children and the husband of a Jewess, you may wish to consider conversion anyway.
QUESTION: "I am a Chinese and I came to US from China 4 years ago. Somehow I was fascinated by Kabbalah, and I have been studying it for three years. I am married to an American Jew but I did not convert. Kabbalah is mystical and my life is mystical. Somehow I realize I am Jewish (past life). And people tell me I am insane.
I am looking for some help, maybe a Kabbalist to help me find out my identity. If I am really a Jew I do not need to convert. I also want to move to Israel to continue studying Kabbalah. Could you give me some advice?"
ANSWER: What an interesting letter! Your problem is based on a simple but 180 degree misunderstanding.
You say: "If I am really a Jew I do not need to convert." The exact opposite is true. If by "am really a Jew" you mean that you have a Jewish soul, nevertheless it is still contained within a non-Jewish body. So according to Jewish law, you are 100% non-Jewish, no matter what your past lives or status of your soul.
The only rectification is conversion, if you sincerely desire and feel the need to be Jewish. On the other hand, if you are comfortable as a non-Jew then there is no need to convert. Except, of course, for the problem that you are married to a Jew, yet his children with you will be non-Jewish too, because the Jewish soul is transferred through the mother.
You report that "people" think you insane. "People" are irrelevant; what does your Jewish husband say?
QUESTION: "What do our sages make of a person who comes from a 100% Jewish lineage on the father's side, and none on the mother's side? Outside of the biological logistics of verifying bloodline that are indisputable if you reckon through the mother, are there spiritual or other reasons why we reckon the child's Jewishness that way?"
ANSWER: We understand that the Jewish soul is transmitted through the mother, and that this is a spiritual matter. Let us remember that blood and DNA are not the same thing as soul, a spiritual essence. Kabbalah gives some explanation of why it is through the mother, which is too technical for this forum of brief answers. Also it is clear that Kabbalah endeavors to explain an existing fact, rather than the reason upon which a decision is based.
One basis for the mystical understanding of the status of a convert is the basic Talmudic ruling on conversion which begins, "When a convert comes to convert..." Shouldn't it say "when a non-Jew comes to convert"? He can't be called a convert till after he converts right?
From this statement we understand that the reason he is coming to convert is that he already has the Jewish soul (although it hovers above him until it enters his body at the actual conversion), and it is this soul which is drawing him to Judaism, whether consciously or not. That is one reason why we are told to discourage conversion candidates initially and make it difficult for them. Those with the Jewish soul will persist no matter what.
QUESTION: "I've read various interpretations on conversion. Some say that the convert was a product of Abraham and Sarah's marital unions that didn't result in a child, other suggest that the converts are souls of those non-Jews that wanted the Torah, but their leaders rejected it. Since the Zohar stresses the concept of measure for measure so much, I've always thought that converts were Jews in previous lifetimes that made some grave religious transgression like becoming an apostate. Could you help sort out the legitimate Kabbalistic approach?"
ANSWER: In most cases it is probably a trapped Jewish soul, as you posited, but not necessarily for the reason that you speculated. Please note that the other theories you raised are not incompatible with this. For a lot more on this subject, keep abreast with our "Gate of Reincarnations" translation.