Final answer:
The acceptance of saints is not a major doctrinal difference between Roman Catholicism and mainstream Protestant denominations. Key differences include the authority of the Pope, the doctrine of transubstantiation, and the nature of salvation.
Step-by-step explanation:
The aspect that is not a major doctrinal difference between Roman Catholicism and mainstream Protestant denominations is the acceptance of saints. While the veneration of saints does indeed differ between these Christian traditions, it is not as contentious as other theological issues.
The major differences include the authority of the Pope, with Protestant denominations rejecting papal supremacy. Transubstantiation is another key doctrinal difference; while Catholics believe that the bread and wine used in the Eucharist become the actual body and blood of Christ, many Protestant denominations view it as symbolic.