In the poem the soul is running errand for the poet. The errand is to convey the poet's valedictory wishes, that is, to go out and tell the whole truth and nothing but the truth to such temporal and spiritual and authorities as the court the potentates and the church as a means of calling them to order.
The poet believes that the court is supposed to be a shining example of justice. However, it seems to be the experience of the poet that the court is both corrupt and unjust. Instead of continuing to hold the scales between right and wrong as the last hope for justice, it ironically "shines like rotten woods".
The message for the church is that as the institution of God Almighty its gospel should be the first and the last word in ethics. According to the poet, however, the church is unfortunately failing like other temporal institutions. This is because it is common knowledge that the church "says what's good and doth no good".
The poet's message for temporal authorities is that they are powerless in themselves. The power they wield is in essence, derived from the people who can also take it away from them. They should not lose sight of the fact that they cannot even pretend to carry everybody along as they are only "strong by a fraction". For this reason, they can only enjoy the people's love and good will if they are also prepared to give them their all.
Finally, these messages portray the poet's experience of the pretensions and inadequacies of the "court", the "church" and the potentates". As he departs the world, the poets earnestly charges his immortal soul to go out boldly and tell the authorities concerned the whole truth loud and clear and damn the consequence.