Countries can't move one from the past if they are enforcing values and ideas that are completely outdated. I have nothing much against religion. I don't care if people are Christian, Muslim, Hindu, Buddhist, etc., but religion should be kept out of the government. I know plenty of theocratic countries, there's Iran, Somalia, Saudi Arabia, Iraq, and Vatican City. Most of them are developing countries.
Separation of church and state is a foundation of liberty. Each person must
be allowed to worship God freely, as he or she chooses, and should also be
free not to worship if that is his desire. When the state promotes a specific
religion, it is likely to discriminate to some degree against people who are
not of that religion, and such people are likely to suffer economically and
socially as a result. That sort of prejudice will be discouraged if the state
stays away from religion.
Given the establishment clause in the first amendment:
"Congress shall pass no law respecting an establishment of religion..."
This has been interpreted that government cannot endorse a specific religion over another. This means government, under the constitution, has basically two options in order to give all religions and belief systems equal footing/opportunity in society:
1) Allow something that applies for all religions/beliefs when government is involved
2) Prohibit something that applies for all religions/beliefs when government is involved
For example: religious monuments in public places. It is technically constitutional to have a Christian monument in a public place, so long as you allow for a Muslim monument, Hindu, Buddhist, etc. As well. With all the monuments you would have to allow and all the backlash from the radical Christian right you might get with, for instance, a statue of Muhammed right next to a statue of Jesus, it is most practical that government simply does not allow religious monuments in publicly funded places and simply allow them to be erected on private property.
Government has to remain neutral with respect to all religions, and it is the best way for a government to run not just for the non-religious, but for the religious as well. The intention of separation of church and state is to make sure that one dominant religion cannot use the government to suppress the religions with a smaller contingency and/or infringe on the rights they have to practice their religion and express their beliefs. In fact, it is the very thing our founding fathers were avoiding by leaving Britain and it is the best way your religious freedom will be protected, especially when your religion is not the majority in the country you live in and/or it is an unpopular religion.
If you really need more reasons why Separation of Church and State is most practical, consider the publicly funded voucher laws that were proposed in Louisiana/Tennessee not too long ago. Politicians in favoring this law had in mind that the money would go to Christian Schools. Yet when find out that Muslim schools could qualify for the voucher money as well, they immediately seek to kill the bill.
Whether or not you are religious, it is not only most in line with the constitution, but most practical that government stays neutral with respect to all religions and remains separate from religion and religious institutions.
Church and state need to be together more because the laws need to be fair and go along with some of the religion. For instance some religions can't do certain things such as cut their hair or wear pants. When our constitution was first made it was based on a Christian foundation. To sum up my thoughts we need the church and state together.