What does John 12:43 mean?

The vast majority of the nation of Israel rejected Jesus of Nazareth as their Messiah. This was predicted by Old Testament prophets like Isaiah (Isaiah 53). Among the religious leaders, most ignored the obvious evidence in order to oppose Him (John 5:31–47). In fact, the leaders of Jerusalem were so committed to rejection of God that they responded to a miraculous resurrection by plotting the assassination of the restored man (John 12:9–11).

Some of those religious elites were more receptive. Nicodemus had visited Jesus to ask more about His teachings (John 3:1–15). Nicodemus also spoke up in favor of giving Jesus a fair and honest hearing (John 7:50–51). However, his peers had made it clear that anyone who openly supported Jesus was going to be excommunicated (John 9:22). So, men like Nicodemus and Joseph of Arimathea (John 19:38–39) kept their faith a secret.

John's condemnation is harsh, but hard to argue with. Sincere or not, neither Nicodemus nor Joseph were willing to step out in full support of their beliefs. Both seem to have done so after Jesus was killed. Joseph might even have spoken up during the sham trials yet to come (Luke 23:50–51). Still, at this time, they opt to maintain their status and the "glory that comes from man" rather than fully following God (Luke 9:23).