Why did God put blindness on the nation of Israel when the Gentiles began being saved?
God placed national blindness on Israel because, when Gentiles began being saved and the church which is His body was being called out, the Jews became envious and opposed the work everywhere Paul went. This national blindness did not remove individual Jews' personal opportunity for salvation.
Les explains that as the church, which is His body, began to be formed predominantly by Gentiles who were being called out, the Jews became envious and began to oppose it everywhere Paul went, so God put a blindness upon the nation. Les reminds listeners that God deals with the Jew on two levels, national and personal. When He blinds them nationally, that does not take away the personal opportunity for salvation. In Acts 15, Peter recounted how God at the first visited the Gentiles at the house of Cornelius to take out of them a people for His name, and James confirmed this calling out of the church.