The tribes of Rueban and Gad wanted the pastures of the conquered Bashan and Sihon because of their vast numbers of cattle after Israel defeated them. Moses warned them of trespassing against God if they lingered in a land without helping their fellow tribesmen possess their own land. Gad and Rueban agreed without hesitation but Moses reinforced the importance of keeping their word.
The sin of doing nothing that is sadly a common thing among Christians and needs to be dealt with. The sin is to forget that the share in the war against evil is a family affair.
Spurgeon says, “This text condemns idleness and self-indulgence. On Sunday some worshipers care only about feeding their souls and Soul-saving is pushed into the background. Unless we shake off that horrible selfishness and feel that the essence of our religion lies in love and that one of the first fruits of it is to care about the salvation of others, then this text solemnly threatens us.”