Before the Vigenère cipher, a simple monoalphabetic substitution cipher was the most advanced encryption. This is a weak way of coding however, as an encryption is only as strong as the key used to create it, and tools such as frequency analysis make this easy to conquer. Any code could be broken if the person who intercepted it was well acquainted with basic deciphering methods. The best way to protect your secret message was to assume that anyone could intercept and decipher your code. It was a given that before the Vigenère cipher was invented, that no encryption was completely safe. That being said, not many people realized this and truly thought they were keeping their secrets safe. A perfect example of this is Mary Queen of Scots.
Mary Queen of Scots spent her time imprisoned sending encrypted messages back and forth with a conspiracy group. Mary, along with the rest of the group, ignorantly thought that no one was able to crack their "master" encryptions. As a result they talked about many sensitive topics, especially the coup to overthrow the Queen of England. Their false sense of security led to their demise because, in reality, their code was very easy to break. They thought that their code was unbreakable, however, there was no sure way to know how accurate this claim was. Mary downfall was underestimating the environment in which she lived. She assumed that no one would be smart enough to break her code, but as she soon learnt, an encrypted message can be cracked to spill the secrets it contains.