As Stephen goes on throughout the novel, he begins to come of age and tries to find himself while struggling with wether he should be the artist he wants to be, or follow the church (which is what his family and friends are basically pressuring him to do).
Later in the novel he has a vision and with this he decides to be an artist. He passes out and when he wakes up he writes a poem. I'm sure he has written more poetry between the first poem he wrote at the beginning of the novel and this one. Although we didn't see any of that other poetry I'm pretty sure this poem was the first one that was truly an authentic poem. The poem he wrote was very real, he didn't try to force this poem out, he didn't try to fit in with this preconceived notion of poetry he had prior to this.
I think this moment in the novel is the real coming of age moment. He really delves deep into himself and finds this poem. He really tries to write it himself rather than trying to write something he thinks he should be writing. This is an important point in the novel and I think that this is Stephen truly becoming a real artist, the moment I've been waiting for throughout the whole book.