The term secular is used to distinguish non-religious things from religious ones. The thing need not be anti-religious to be secular; in fact, that would be a misleading use of the term. Instead, describing something as secular generally means it is simply unrelated to religion.
For example, the United States government is, under the Constitution, a secular government. That does not mean the government acts against the interests of religion or the religious. On the contrary, it means that the government must remain neutral with respect to religion, to the extent possible. Furthermore, it doesn't mean that religious people cannot populate government positions. Indeed, in the U.S. most office holders are religious. But government officials cannot use their executive, legislative or judicial powers to promote one religion over another, or religion over irreligion.
"Is there any maxim in politics more certain and infallible, than that both the number and authority of priests should be confined within very narrow limits; and that the civil magistrate ought, for ever, to keep his fasces [bound wooden bundle symbolising judicial authority] and axes from such dangerous hands? But if the spirit of popular religion were so salutary to society, a contrary maxim ought to prevail."
"History, I believe, furnishes no example of a priest-ridden people maintaining a free civil government."
- "We have to reappropriate the concept of laïcité [secularism] so we can explain to our young pupils that whatever their faith, they belong to this idea and they’re not excluded. Secularism is not something against them; it protects them [...] If a big number of young pupils felt secularism was an attack on them, it was because the term had been misused and deformed in the public debate for years by the extreme-right and the right as an attack on Islam. "
An argument against secularism is that the loss of religion is causing national decline. Another is that "turning our back on God" will leave us unprotected against disasters or God will actively causes disasters as punishment.
Secular vs. atheistic
Being secular is not the same as being atheistic. Both believers and non-believers (with respect to any particular god claim) can participate together in a secular purpose.
Secularism causes extremism?
Some apologists have argued that as religion becomes marginalised, it becomes more extreme.
- "I believe the marginalisation of religion in our national life risks pushing more young Muslims into the arms of Isil. ... This is where hard-edged secularism ... actually serves to aid and abet the extremism we are all seeking to confront. Because if you push faith to the margins, then to the margins and into the shadows faith will be outworked "
- "It is driving people underground and for young people with an interest in spirituality it does make them vulnerable to grooming by extremist groups. "
An opposing view is that moderate religion legitimizes fundamentalism.
Secularism and quality of life
Many of the most stable and high quality of life countries are very secular. Of particular note are Sweden, Germany, Spain and France which are highly secular. Some of the lowest quality of life countries are strongly Christian, including the Dominican Republic and El Salvador.  More strongly religious states within the US have a lower quality of life (Mississippi and Alabama). More secular states in the US have a higher quality of life (Vermont and New Hampshire).
- "if declining levels of religiosity were the main cause of…social ills, we should expect those countries that are now the least religious to have the greatest problems. The reverse is true."
Apologists sometimes argue that secularism causes societal problems. This is largely due to their dogma that morality depends on religion.
- "Today, countries where freedom, justice and tolerance are the norm are almost all influenced by the Bible's teaching, in stark contrast to those that have deliberately rejected it."
Apart from being false, it also this overlooks the millennia of conflicts between Christians, the Crusades, the Inquisition, etc. Many of the conflicts in the world are caused by imperial ambitions of majority Christian countries; either the West (Catholic/Protestant) or Russia (Orthodox Christian). Just because the Bible had some influence, it does not make it the main influence in personal or political freedoms or security; in fact it appears to have a negative influence.
Government as a replacement for God
Some theists argue that reliance on secular government replaces our "reliance on God" and that it is a form of idolatry. This argument is best countered by rejecting the premise that God is something to be relied on.
- "In effect, we allow or permit or volunteer ourselves to become government slaves and they become our masters and thus we lose our sovereignty and thereby make God of secondary importance, presumably because we want a hand-out and government “security”. [...] By surrendering our sovereignty and letting government become our god, we have committed idolatry: relying more on government and man than we do on God or ourselves to meet our needs."
- Rahman, T., Mittelhammer, R., & Wandschneider, P. (2011). Measuring quality of life across countries: A multiple indicators and multiple causes approach Journal of Socio-Economics, 40 (1), 43-52 DOI: 10.1016/j.socec.2010.06.002
- John Blanchard, Why believe the Bible?, 2004