Allah knows best how to weigh all aspects of a person's nature, intentions, capability, understanding, opportunities, knowledge, weaknesses and so forth. So, in the next life, Allah will judge absolutely justly, and no one will look at how Allah judges in the next life and say that Allah has been unfair. We will also understand better why people are judged as they are, since sometimes a small action may lead a person to jannah and someone who seems good on the surface may end up in the other place.
However, having genuine wilayah (allegiance) to the Imams will, inshallah, aid a person in helping to attain a higher spiritual station, benefit from their intercession, and allow us to be with them in the next life.
Most non-Shi'a today are not directly rejecting the Imams; rather, they are just following what they grew up with or what society says around them. This is different from someone who, for example, rejected Imam 'Ali (A) to his face. As is narrated from Imam Rida (A): "May Allah have mercy on the one who keeps our [teachings] alive... for if the people knew of the virtues of what we say, they would have followed us."
It is important not to be arrogant in one's faith, since even if a person believes in the Imamate, they may still suffer from misconceptions about what is true and be informed in the next world about those things that they thought were correct, which were not. Inshallah Allah would also be merciful about that. However, we can, God willing, pray that Allah guides us to what is most true.
