The poem "A Poison Tree" can be read as a comment on forbearance. More specifically, this is a poem about the relative value of forbearance versus the expression/exorcising of negative emotions.
To put the poem into a Christian context, we might argue that the poem's commentary addresses the notion of "turn the other cheek" forbearance of grievances, suggesting that stifling one's negative feelings will only lead to a festering and growing negativity inside the individual.
"Burying anger rather than exposing it and acknowledging it, according to "A Poison Tree," turns anger into a seed that will germinate" (eNotes).
The opening stanza of poem juxtaposes an act of expression of negative feelings with an act of forbearance.
I was angry with my friend:
I told my wrath, my wrath did end.
I was angry with my foe:
I told it not, my wrath did grow.
The dichotomy established here is carried through the rest of the poem. The speaker's wrath remains potent and even increases with time until it finds expression.
In the morning, glad, I see
My foe outstretched beneath the tree.
"A Poison Tree" speaks to Blake's philosophical outlook wherein that which encourages or articulates energy is virtuous and that which restrains or constrains energy is vicious (e.g., a vice/bad).
The implicit metaphor of the poison apple in the garden connects the poison fruit to the tree of knowledge of good and evil from the Garden of Eden story. In drawing this connection, Blake's poem creates a suggestion that the enemy comes to know the speaker's wrath and thus that wrath is communicated directly to the enemy.
In this event, there is no turning away from the foe but rather a celebration of the fact that, in the end, the foe has come to fully understand the speaker's feelings. (There is a hint available in the poem that if the enemy had understood the speaker's negative feelings before stealing into the garden, he may have been wary of eating the fruit therein and been saved.)
In the end, the truth is exposed and this inspires gladness in the speaker of the poem. The moral or philosophical sentiment of the poem then is also related to a notion that honesty and truth should be placed above gentleness if that gentleness is, in fact, dishonest and dissembling.