I have been one acquainted with the night (1 and 14)
This line personifies an element of the natural world. Though acquainted can be used with objects, not just humans, it's more common to refer to being acquainted to a person than a time of day. So, in this lonely poem, we see that the speaker places emphasis on his connection, however ambivalent, with the night and the natural world.
I have walked out in rain – and back in rain (2)
Here, the natural world is not being very kind to our speaker. We deal with a negative aspect of nature, the rain. Yet the speaker doesn't seem to be afraid of the rain. It's just another thing he deals with in the course of the night.
I have outwalked the farthest city light (3)
This line starts to show the contrast between man and the natural world. The speaker has walked past the farthest city light and beyond the reaches of man, into the depths of nature and night.