Not fasting Ramadan
Fasting in the month of Ramadan is the fourth pillar of Islam. Muslims experience this month as an important moment in their lives.
A Muslim should fast this entire month if he or she is able to do so. Anyone who deliberately leaves this month and does not participate is committing a great injustice.
According to the correct view, it is disbelief to consciously abandon a pillar of Islam. So also leaving the month of Ramadan is one of the acts of disbelief.