Clash of intellect and faith
This theme is most dynamically apparent in the relationship of John to Gabriel, and the relationship of Richard to everyone else.
Gabriel is the strongest and most authoritative voice in John's upbringing. Sadly an abusive stepfather to John, Gabriel also stands as the voice of religion in his life. Accordingly, John associates religion with Gabriel's rages and tempers and lack of love. In his eagerness to dissociate himself from Gabriel, John turns to his intellect, which has been publicly acknowledged even by white teachers. He trusts his intellect to win him success and popularity-to prove to Gabriel that white people and life itself really are not as dire and vicious as he thinks. Gabriel doesn't seem to recognize John's intelligence at all, but Gabriel's approval (or lack thereof) ends up being irrelevant to John's conversion experience. Accordingly, it's interesting to conjecture how John might yoke intellect and faith in his life as a believer after the close of the book.
Richard, on the other hand, is an interesting foil to John here. Richard cleaves to his intellect as a means of proving to white people that he is their equal in intellect. Yet as formidable as his knowledge and intellect is, it isn't enough to convince the white policemen and storeowner to respect him. The accusation of thievery and the subsequent treatment he suffers destroys the frail self-respect he'd built up, and there is no faith to elevate his self-worth. It destroys him, and despite the real love he and Elizabeth have for each other, it is insufficient to keep him from suicide.
African-American religion / Pentecostal religion
Religion is a major theme in Go Tell It on the Mountain. References to the Bible are liberally woven throughout the text both in explicit quotes (as in Gabriel's sermons) and in subtler parallels between different characters in the book and characters in the Bible (as in the parallels drawn between Abraham as the father of the Jewish people, and Gabriel as the founder of a royal line). The effect is to elevate stories that might be taken as mundane or even sordid (affairs, illegitimate children, etc.) into stories with Old Testament repercussions and overtones.
The use of religion also invites the reader to reassess religion. In some ways, religion is portrayed negatively, most obviously through Gabriel. But it is portrayed more positively through John and Elisha, and to some degree Elizabeth. Baldwin never makes it easy for the reader to decide whether religion is negative or positive for any one character or another, but the way Baldwin brings religion into the everyday life of his characters raises questions about the role religion might play in the reader's life.
Also, in the novel faith provides community over something that cannot be taken away--which is particularly powerful and poignant in the context of African-Americans in the 1930s in Harlem. Particularly in the vision John sees of the armies of the dead traveling toward God, religion provides a reason for pressing on together despite the unique hardships and struggles African-Americans faced. It also is something that is inaccessible and incorruptible by white people.
Suffering
Everyone in this novel suffers, and in most cases they suffer a great deal--whether through their own actions or the actions of others.
John suffers growing up with Gabriel as his stepfather. Gabriel suffers through his own pride and anger, and causes so much suffering to the women in his life through it: his mother, Florence, Deborah, Esther, and Elizabeth. Elizabeth suffers through Richard's death. Richard suffers through his betrayal by white people in spite of the lengths he'd gone to. Florence suffers through her pride and ambition.
The only time suffering is truly resolved is in the church, when the saints come into the presence of the Holy Spirit and bring their troubles to him. It transformed Elisha and John so they are joyful, though their youth is certainly contrasted against the wisdom and experience of the older saints.
Guilt
As well as suffering, nearly every major character bears some burden of guilt.
It is interesting to note that Gabriel finds it relatively easy to let go of his guilt, considering himself forgiven and saved by the Lord. Yet signs of his guilt follow him: Royal's death; Florence's continued reminders of his mistreatment of Esther and Deborah; the failure of his legitimate sons to take up the faith; Esther's letter; Deborah's letter to Florence; his children's hatred toward him; the ease with which he fell into his affair with Esther; and his ensuing guilty conscience toward Deborah.
Florence hides her guilty conscience about Frank and how she treated him (and simultaneously her attitude toward fellow African-Americans) by isolating herself (as shown by the way she carried herself at work) and focusing on how much she loved him in her conversations with Elizabeth.
Elizabeth feels guilt about her failure to tell Richard about her pregnancy.
Racism
The theme of racism centers on the character of Gabriel, who makes the biggest deal about it despite not (in the book anyway) experiencing it as severely as Richard. Baldwin is careful to show us early in the book that Gabriel is consistently angry at white people and does his very best to train his children to feel the same way. When Roy gets stabbed in a fight with white people, Gabriel takes the opportunity to pointedly tell John that that's what white people do to black (assigning no blame to Roy for causing trouble).
Yet it is Richard who suffers the most. Unjustly accused of a robbery he did not commit, Richard sinks into despair over the unfairness of it--particularly when he'd gone to such lengths to improve himself. Richard never tried to court white favor, but he did educate himself so he knows he is better than whites. It is not enough to keep him afloat, however, and he commits suicide.
Sex
Before he was saved, Gabriel spent all his time getting drunk and going to prostitutes. Despite his mother's best efforts, Gabriel is unable to stop this behavior. He falls into old bad habits again and again until he experiences salvation. It is enough, for a while, until he meets Esther. Although he lays the blame primarily on her for tempting him, he is attracted to her and they have a passionate 9-day affair until he feels remorse and calls it quits.
John is also the product of an affair, though it's important to note that love actually played a role in his conception, not lust.
There are also homosexual elements, as John feels an undefined attraction toward Elisha.
Blame
Florence blames Frank completely for the failure of their marriage. Next to his inability to spend money wisely, the company he kept, and his drinking, she doesn't see the role her rigidity, anger, and discontent played.
Gabriel blames everyone but himself (and his legitimate children) for the ills that happen to them. He blames Esther for tempting him, Esther's wanton-ness for Royal's death, Elizabeth's affair for Roy's recalcitrance and hatred toward him, Deborah for her lack of sexual passion and her failure to give him a son, and the white people who stabbed Roy despite being provoked by Roy.
Among other things, the use of blame, besides being distasteful in and of itself, casts Baldwin's concept of religion into a new light. Even the "holy people" are just as flawed and sinful than sinners (and arguably in the case of Gabriel, more so). Yet religion itself isn't portrayed negatively--it's the people who participate in it that can mess it up.
