Go Tell it on the Mountain begins with John Grimes' thoughts and descriptions of those around him. His first considerations are concerned with the expectations of his community, which urge that he should be a preacher like his father, and how he had only begun to think about it on the morning of his 14th birthday. This is followed by an introduction to his siblings. His younger brother, Roy, and sisters, Sarah and baby Ruth are referred to in his thoughts as he recalls memories of the whole family going to church every Sunday morning. John also switches to the present as he contemplates the probability that his mother, Elizabeth, is pregnant again.
The narrative shifts back to John's memories of visiting church and the people they pass on the way. The sinners of the neighborhood are described and Roy is drawn in contrast with John as sexually knowledgeable, whereas John is older but more innocent.
The overview of the journey to church every Sunday culminates in an introduction to the Pentecostal store-front church the family attends, which is the Temple of the Fire Baptized. An account of Sunday school is given and Brother Elisha, the 17 year old nephew of the pastor who teaches John and Roy in the Intermediate Class, is described. It is suggested that John has been distracted by Elisha as though he is infatuated. John remembers that one Sunday Elisha and Ella Mae were uncovered in church by Father James for 'walking disorderly'. They had not sinned, but were considered in danger of 'straying from the truth'.
It is fully revealed at this point that these thoughts have been occurring with John still in bed on the morning of his 14th birthday, 'on a Saturday in March, in 1935'. He feels guilty for the 'sin of the hand' he committed (that is, masturbation) at school. He thinks of this whilst looking at a stain on the ceiling. His thoughts then turn to wondering if anyone has remembered it is his birthday as it has been forgotten before.
It is emphasized how conforming and well-behaved he is with the references to his performance of chores and how he excels in school. Both white and African-American teachers have praised him for his intelligence. However, he also yearns for popularity. His father, Gabriel, calls him ugly and he has no friends in class. John's strong feelings against his father become more evident as John wishes for the day when he can curse him on his death-bed.
Once out of bed, he has breakfast with his mother, Elizabeth, sisters and brother. In Gabriel's absence, Roy argues with their mother and says how he does not want his father to beat him. John thinks once more that Elizabeth has forgotten his birthday and vows not to mention it. He cleans the front room and dusts as required by her. It is explained, whilst John is looking at the family photographs, that Gabriel has been married previously and his first wife, Deborah, died.
Before leaving the house, John's mother gives him a little money to buy himself something for his birthday. John goes to his favorite hill in the middle of Central Park and feels a sense of power as he runs up it. His ambition to be the 'mightiest' is immediately deflated when he pauses to think of how he is also a stranger in his own city. His theological torment is exposed as he questions the 'narrow way' of his faith, which has given him 'a belly filled with wind and had bent his mother's back'. The glories of the present are compared with the glories of eternity as he struggles with temptation for material comfort and pleasure and the eternal after life offered by the narrow way of the cross.
Gabriel's views on the wickedness of white people are explored as John juxtaposes this perspective with his own experience of a kind white teacher. He also remembers what he has read about the treatment of African-American people in the South, and the violence inflicted on them by whites. John then becomes fearful to enter a shop frequented by whites and dare not enter the public library even though he knows he is entitled to. He decides to visit the cinema instead with the money given to him. Here, he identifies with the main female character who is blonde and pasty white. This connection comes about because he, too, wants to tell people how he hates them as she does.
On his late return to the family home, John is chastised and discovers that Roy has been stabbed. His parents and Aunt Florence are caring for Roy as he enters. In the ensuing arguments, Gabriel slaps Elizabeth and Roy calls him a 'black bastard'.
John then goes to church to clean it. Elisha enters and John argues playfully with him. This lively exchange between them climaxes in a friendly bout of wrestling. It is also mentioned here how John is not saved yet. Two of the saints (saved members of the congregation) come into the church and these are followed by John's parents and Aunt Florence. John notes how Florence has not attended this church before.
Written in the third person, Part One has its central focus on John Grimes. He is established as a sympathetic character. He is an adolescent who feels alienated from those who would be expected to be closest to him, and as this section proceeds it becomes apparent how his father dominates not only John, but also the rest of the household. Central to the introductory part of this section is the extent of the expectations that are placed on John, such as following Gabriel into the church to be a leader of people. Accompanying this weight is his sense of isolation from everyone around him. This is made clear through the symbolic use of his birthday and the likelihood, in his belief, that his family has forgotten about it.
The reader is privy to John's perspective predominantly in Part One and the reasons for Gabriel's cruelty and violence are not explained at all at this juncture. This perspective is, therefore, compromised by John's subjective position. It is evident, however, that Gabriel rules the family as the unquestioned patriarch and both his sons, John and Roy, despise him for this.
John's antagonistic relationship with Gabriel disturbs his religious beliefs, which, in turn, reinforces the theological struggle that is a main element of this novel. John's self-torment is recognizable in his self-loathing and this appears to be reinforced by the cruelty of the father. It is also an aspect of his religious faith when, for instance, he feels guilt for what he believes is the sin of masturbation. His alienation is reinforced by his father and his religion, although the novel tends to critique Gabriel more than the religious ideology he has been raised with.
This ideology is critiqued briefly, however, in the crucial paragraphs where John stands on his favorite hill and swings between desires to be ambitious and materially content, or to live on in the eternal after life. His perspective touches on criticizing the 'narrow way' of his religion that has only entailed poverty, hard work and hunger and has kept people in their degraded place in society's hierarchy. Following his thought process, this way of living in the future, with regard to the after life and avoiding the temptations of enjoying the present, is connected to the fear of entering shops and a library in a neighborhood dominated by white people. Gabriel's passionate hatred for whites, which has been vindicated by knowledge of the treatment of African-American people in the South, is considered by John and not fully discarded. Ultimately, though, he believes that one day he will not be afraid to enter these buildings and offers a somewhat naive view that appears to still question Gabriel's experience of the South because he despises Gabriel and his beliefs.