Change/declare a constant in setup and use it in loop
You are not going to get good answers without showing your code, but maybe this is a good guess of what you want:
why not try something roughly like this (untested and incomplete)?
Adafruit_BMP3XX bmp;
float startPressure;
float difference;

void setup() {
  bmp.begin_I2C();
  startPressure = bmp.readPressure();
} 

void loop() {
  difference = startPressure - bmp.readPressure();
}

Or, better:
Adafruit_BMP3XX bmp;

void setup() {
  bmp.begin_I2C();
} 

void loop() {
  static const float startPressure = bmp.readPressure();
  static float difference;
  difference = startPressure - bmp.readPressure();
}