When did God plan to bring the Gentiles to a knowledge of Him?
God planned to bring the Gentiles to a knowledge of Him at the future time when Israel would be funneled back into the Gentile nations. The Gentiles were outside the covenant promises, without hope and without God, until that appointed time, which Old Testament prophecy foretold.
Les teaches that after calling Abraham and establishing the Abrahamic covenant, God called Israel out of the mainstream of humanity to become a kingdom of priests. To prepare them, He put them under the law, gave them temple worship, the priesthood, and the Old Testament. Meanwhile, the Gentiles were allowed to go their own way, outside the covenant promises, not citizens of Israel, without hope and without God. Les explains they continue until such time as Israel can be funneled back into the Gentile nations with the idea of bringing them then to a knowledge of their God. This is the whole scope of the Old Testament, which is prophecy, telling future events, including Christ's death, burial, and resurrection, though in subdued and latent terms nobody understood.