One of the reasons stress damages the stomach lining is thought to be as follows.

Literal

Stress [subject-が] stomach [possessive-の] mucous-membrane [object-を] damages reason [possessive-の] one [topic-は] following-as is-considered.

~のひとつは (one of) introduces a specific example from a larger set. ~ように考えられている is the passive of ~と考える, meaning 'is thought to be' — a standard academic hedging construction. 粘膜 (mucous membrane) is medical terminology.