I have heard that Rashi said that there are three requirements for conversion: brit milah; mikveh; korban in Beit Hamikdash. If this is true, why is it that this is not rendering modern conversions invalid until the Beit Hamikdash is rebuilt?
This question is related to another and I believe the answers are the same. The sugya (discussion) is found in Talmud Bavli, Krisus 8b-9a. The resolution there is essentially that the korban (offering) is not m'akev (does not invalidate the conversion), which is learned from a gzeiras hakasuv (exegetical interpretation of a verse).