Ignazio —
Mortal sins require forethought and deliberation. What you did was an involuntary, or at least an impulsive, reaction, so don't worry about it being a mortal sin.
You could tell your friends that God means a lot to you and you'd appreciate it if they did not speak disrespectfully of Him, just like they wouldn't appreciate it if you spoke disrespectfully of one of their friends.
They may make fun of you, or there may be other consequences, but your reward in Heaven will increase if they persecute you as a result.
Eric |