"Allah knows what is in my heart".... how many times have you heard this from someone who is trying to make excuses for why they don't follow a part of Islam? In one of my classes our teacher brought this up and then reminded us of Surah Al Asr
Indeed, mankind is in loss,
Except for those who have believed and done righteous deeds and advised each other to truth and advised each other to patience.
So you see, the Quran is telling us that we must do righteous deeds, and advise each other to the truth. Islam is a religion of inner and outer faith. We must believe it in our heart and show it with our actions.