Why should you not believe in God when the messenger is inviting you to believe in your Lord? He has taken a pledge from you, if you are believers.
وَما لَكُم لا تُؤمِنونَ بِاللَّهِ وَالرَّسولُ يَدعوكُم لِتُؤمِنوا بِرَبِّكُم وَقَد أَخَذَ ميثٰقَكُم إِن كُنتُم مُؤمِنينَ
wa maa lakum laa tu’minoona billaahi war Rasoolu yad’ookum li tu’minoo bi Rabbikum wa qad akhaza meesaaqakum in kuntum mu’mineen