"A country like India will not become strong merely by increasing number of submarines, missiles or bombs but further strengthening unity amid diversity within," Kumar told an election rally in minority-dominated Bhagalpur.
"India is full of diversity where people practice different religion, speak varied language and live in dissimilar geographical area," he said.
"When crores of Muslims get frightened in his name how come he can make the country strong," he said in an apparent dig at Narendra Modi.
"A leader who could not keep the country one can never do good for the country," he added.
Kumar, who is senior leader of JD(U), justified breaking away from BJP over elevation of Modi.
"When we realised that BJP in its new incarnation has diverted from the path of Atal Bihari Vajpayee and going back to controversial agendas of constructing Ayodhya temple, scrapping Article 370 that gives special status to Jammu and Kashmir and imposing uniform civil code, JD(U) decided to say goodbye to them," Kumar said.
Bhagalpur, which goes to polls on April 24, is witnessing a triangular contest between BJP's Shahnawaz Hussain, JD(U)'s Abu Kaiser and RJD's Bullo Mandal.