The plenary indulgence which was granted to Crusaders was not a promise of salvation and entrance to heaven. Under Catholic doctrine, salvation and entrance to heaven could be achieved only through Christ. Indulgences, plenary or otherwise, were not guarantees of salvation. What they did was let you off time in purgatory prior to your salvation. In other words, they didn't secure forgiveness for your sins; they let you off (some of) the punishment you were due for the sins that God had already forgiven you for. (This was because they thought that even though God had forgiven you your sins, you still had a bit of purging due to you.) If you weren't going to be saved anyway, you wouldn't be going to purgatory in the first place, and an indulgence wouldn't do you much good.