Why do Muslims try so hard to dispel Christianity?

The muslim clerics realise that Christianity is the only true religion. However, they cannot let the cat out of the bag as muslims would leave Islam and turn towards Christianity for they consider Christ as one of their prophets. Who will then support the clerics and give them the power they are used to? Therefore, the best defence is to attack the truth and try and turn people against the truth. That is why the attack on Christianity only and no other religion.