There is statement which says, "They are not successful, a people who only know Allah in Ramadaan." Is this Hadeeth authentic?
This statement is from some of the Salaf with regards (or in response) to being asked about a people who worship and strive hard in Ramadaan but once Ramadaan leaves they leave off (worship and striving hard). Thus, he said, "Most despicable are a people who only know Allah in Ramadaan." This is correct if they abandon the obligatory deeds. However, if this is not (the case), and rather they leave off some of the extra efforts, then this statement (in regards to them) is not correct. Therefore, what is intended by it, (is in regards to those) who abandon the obligatory deeds. Meaning, (they) pray in Ramadaan but leaves off the prayer in other than Ramadaan, for example; these are a most despicable people. This is because, they would have disbelieved (by way of this action). The abandonment of prayer is disbelief, we ask Allah for safety.
However, if they leave off some of the voluntary deeds in outside of Ramadaan, then this would not harm them. This is due to the fact that people (tend) to strive harder in Ramadaan in (doing) various types of voluntary deeds and different forms of charity. If they become lax in those voluntary deeds after Ramadaan, then it is not said about (such a people), "A most despicable people..."