The Godfather: A man's word is often more valuable than money.

For me, the most important theme in The Godfather comes from the issue of loyalty: a man's word is often more valuable than money. 

The importance of this theme becomes obvious in the very first scene, where Bonasera asks Don Corleone to help him get revenge for his daughter's rape. The favor, however, is worth much more than money; it costs Bonasera his loyalty. This means that now, if Vito asks something of Bonasera, he will have to comply. 

The opening scene isn’t the only case when loyalty acts like currency in Coppola's film. When Salvatore Tessio betrays the Corleone family, his move being “simply business”, he asks Tom Hagen to get him off the hook. However, since loyalty can’t be bought, Hagen refuses and orders to assassinate Tessio.