Ein Sof isn't mentioned in post 48. Ein Sof is a Kabbalistic term and we've mentioned many times that Kabbalah is not a topic to be discussed in this forum (one must first truly understand Torah and Talmud at a p'shat level before even considering to understand deeper levels). Having said that people misunderstand the concept of Ein Sof (“Infinite One”).

Muchrach HaMetzius means that G-d is a necessary Existent. In other words, it refers to the fact that G-d must exist in any conceivable scenario we can think of -- yet creation --everything -- exists because G-d happens to have created it. G-d could have just as easily not have created anything.

סופי

And everything that Sarah tells you, listen to her voice. Bereshit (Genesis) 21:12