Why do we disagree with X and call them heretics?

FINDING THE RIGHT HILLS TO DIE ON:
The case for theological triage
By Gavin Ortlund
Crossway / TGC. 152 pages. £11.99
ISBN 978 1 433 567 421

This is a brilliant book on a vital subject. On which doctrinal hills are you prepared to die? Can we rank Christian doctrines from top-flight essentials through to disputable and insignificant?

I was taught – and have often quoted – the adage attributed to Augustine, or Meldenius, or various others: ‘In essentials unity, in non-essentials liberty, and in all things charity.’