We believe what you have been devoted to----sure to come tru

这里what you have been devoted to=the thing that you have been devoted tothat you have been devoted to 是定语从句,修饰the thing。把that you have been devoted to 这个定语从句去掉,整个句子就变成：We believe the thing is sure to come true in the near future。这里的结构是：We belive+宾语从句。在宾语从句中,主语是以what引导的主语从句,为单数形式（=the thing that you have been devoted to）,所以,be sure to为宾语从句的谓语动词,要用单数形式is。