It would be incorrect to view the terms sectarian and secular as complete opposites. In some cases they can be opposed to each other. However, secular folks may also be sectarian under certain circumstances. Thus these words are semi-related, and may represent opposition or togetherness.
Sectarian tends to be defined as a particular sect, often religious. For instance, sectarian tensions between Catholics and Protestants in Ireland has resulted in numerous years of not only bad feelings, but also violence. Sectarianism includes the idea that the particular sect to which one belongs is the right and proper one, and that others belonging to other sects, even if they’re still of the same overhead religious group have it completely wrong. Though Protestantism and Catholicism are both Christian, when people believe one sect is better than the other, or that one sect is more right than the other, sectarianism may take over and cause discrimination and violence.
Sectarian does not necessarily refer to sects of a religion (Sunni, Shiite or Protestant, Catholic). It can also refer to sectors within a society or to those who ascribe to non-religious beliefs. For instance groups of atheists might be called a sect, or a caste in India might be considered a sect. This is when the terms sectarian and secular can become interrelated. Secular is defined as not religious, not pertaining to a church, or layperson status within a religion. It is non-religious and you can use the terms sectarian and secular together to suggest a non-religious group or sector. A secular sectarian could be someone who does not ascribe to a particular religion and belongs to a sect of people who share these beliefs.
Furthermore, when religious tension between two groups is particularly high, sectarian and secular may be related to denote that the religious leaders of either group do not condone the actions of laypeople within a sect. A religious leader could denounce violence between two groups as secular sectarianism. This has certainly been the case with many religious leaders in Islam of both Sunni and Shi’a sects.
Though there are some religious leaders who condone and encourage violence between the two sects of Islam, other leaders, Imams, argue against continued sectarian violence and blame this behavior on “secular” or layperson misunderstanding of the Qu’ran. They relate the terms sectarian and secular by suggesting that those who haven’t studied and become religious leaders are secular sectarians who have no right to speak for the religion at large.