God choosing us though did not involve our own preexistence, but this speaks to God's foreknowledge, as God the essential being of existence has no limits to anything, including God's knowledge, which is then unlimited. God knows all, but this knowledge of us is not the same as other knowledge, which is knowledge about things. There is a word of another knowledge, which we do not have in english, that it is not indicated in translations, the personal knowledge. God knows already those who do come to God in a relationship when they do respond to God in their own lives. So it is that Jesus spoke of others who will be told, "Depart from me, I never knew you." They were never known personally, as they never have the personal relationship with God that people should come to, that since the fall of humanity has been made available through Jesus Christ.
Indeed that there is a time that everything of physical existence began indicates that there was the essential being of existence that made it, this was the Creator, the always existing unlimited one.