There are some acts that could nullify ones Islam. First, to perform shirk in worshipping Allah. Second, People who create middlemen/mediators between themselves and Allah. They pray and ask for Shafaat (intercession) to the middlemen. They also trust those mediators. They are considered as disbelievers, based on the consensus of scholars (ijma). Third, Unwilling to consider those who worship other but Allah as disbelievers, hesitant towards their disbelief, or justify (on their sides) their idea.
In a book of Shekh Muhammad Ibn Sulaiman, which named "Nawaqidul Islam", ten points were mentioned as the nullifiers of Islam. Now, you can learn about the ten points by using your iPhone or iPad. Just buy the application "The Nullifiers of Islam" that published by Yufid, Inc. May you get more knowledge about Islam as the best guidance for your life.