Al-Munafiqun 63:3

Thalika bi-annahum amanoothumma kafaroo fatubiAAa AAala quloobihim fahum layafqahoon

That is because they believed, and then they disbelieved; so their hearts were sealed over, and they do not understand.

That is because they declared faith (in Islam apparently), then disbelieved (secretly). Therefore a seal has been set on their hearts, and thus they do not understand.