Found a solution, if anyone ever has a similar problem.. I changed

passport.deserializeUser(function(id, done) {
User.findById(id, function(err, user) {
  done(err, user);
});
});

to

passport.deserializeUser(async function(id, done) {
try {
  const user = await User.findById(id).exec(); 
  done(null, user); 
} catch (err) {
  done(err, null); 
}
});

And added {} to arrow function before res.render(which wasn't the only problem)