“My kindest friends were never Muslim. Am I supposed to believe they are all going to hell?”

Nobody is in a position to tell you where your friends are going, and anyone who claims to be is overreaching.

Judgment belongs to God. No teacher, no relative, and no writer has been given the roll. The Qur'an is explicit that there is no compulsion in religion, at 2:256, and it is equally explicit that the reckoning is God's alone. A person who assigns your friends to the Fire has assumed an authority that was never delegated to them.

The category the polemic turns on is kafir (one who rejects the truth), and it is not a synonym for "non-Muslim". It describes a posture toward truth that is knowingly refused. Whether any given person occupies it is not something a stranger can determine, and it is not something you were ever required to determine about the people you love.

It is also worth noticing what the objection quietly assumes: that the non-Muslim friends were the good people, and the Muslims were the bad ones. That may be an accurate report of a particular childhood. It is not a fact about two billion people, and a conclusion about a faith built on a sample of the individuals who were unkind to you is a conclusion about them.

You can hold your friends in the highest regard, be grateful to them, count them among the closest people in your life, and remain a Muslim. Nothing in the religion requires you to choose. Anyone who told you otherwise handed you a false choice, and you are permitted to hand it back.