The poem is short and strange, and its meaning shifts depending on how you read "the gun." The first line sets a gun beside "an excellent doctrine of St. Peter," so religious teaching and the threat of violence sit side by side with no explanation. The poem never says whose gun it is or why it's there.
What the speaker does next is the unsettling part. They "spur" themselves to be afraid, as if fear were a horse to be urged on, something chosen rather than simply felt. They also decide to hear little and to "understand without going to see." That reads like a kind of obedience: accept what you're told, don't investigate, don't look too closely.
The mood is tense and quietly uneasy. Judging by the wording, the speaker seems to treat fear and blind trust as a survival strategy, or perhaps as a doctrine of their own. The poem leaves it to us to decide whether that is wisdom or surrender.