Greetings of Peace. I do think that it is possible for a Muslim to have a Christian teacher living or dead but the latter situation proposes certain difficulties that must be given full and serious consideration especially in light of your friend’s response to your experiences. The first thing to consider is that esoteric instruction from a Christian would not constitute Sufism as it is traditionally understood. It would not necessarily be considered herterodox, for esoterism is intrinsically orthodox, but neither would it belong to the traditional path of Sufism as it has been passed on throughout the centuries. To my knowledge, those who do learn from "foreign" masters also obtain an attachment at some point from a representative of their own tradition.
Overall, your primary aim seems to be the authentication of your experience. You wonder if you did in fact receive authentic spiritual instruction from Elder Ieronymous of Aegina through dreams. Your friend replied that only you can tell yourself if it was real. He is correct in that the capacity of judgment is incumbent upon everyone and also that despite whatever conclusions anyone else may propose, you may still agree or disagree. Nevertheless, judgment is one thing and correct judgment quite another. To assert that one’s own judgment determines the authenticity of the experience is to fall into the subjectivist error and abolish the recognition of objective truth. If you need further clarification, please let us know, as this is a very important point and failure to understand it can lead to many abuses and stray roads. Many false paths outside of the Islamic world and as I am slowly coming to learn, inside it as well, are based upon the idea of hidden masters whose existence is corroborated by dreams, visions, and fantasies which are uncritically accepted by recourse to the the subjectivist position.
I do not wish to reject a priori, either your experience or that of the hidden master, both of which I accept as legitimate possibilities. I do, however, recognize that such things are potentially subject to both self-delusion and abuse and so recommend that you seek the counsel of a living spiritual master who is qualified to aid you in determining the authenticity and level of significance of your experiences.
Greetings of Peace. This blog contains selections from my correspondence and other sources on a variety of subjects related to religion, philosophy, and spirituality. I hope that they may be of benefit to the interested reader. Concerning the title of the blog, read this entry.