Allah is the Arabic word for God, in the same way that Elohim is the Hebrew word for God and Dieu is the French word for God. Arabic-speaking Christians pray to Allah. Therefore it is incorrect to say you believe that "Allah is not the true God." You could say you believe that "the Muslim God is not the true God" or "the Muslim Allah is not the true Allah" or "the Muslim Allah is not the true God" etc.