I’ve written about secularism in the past, but Qualiasoup has a straightforward video explaining it, if that’s your preferred medium. I think the most important thing to stress is that there are both atheist and theist secularists. There are nothing but good reasons for both sides to come together in this, as a secular government protects the rights of all people, regardless of religious belief.
The opposite of secularism is theocracy. The problem with this, as I have said before, is that when anyone succeeds at implementing anti-secular legislation, it hurts everyone who is not specifically a member of the elevated religious sect. No matter what denomination of whatever religion you belong to (if any), the odds do not favor you in this (and the desire to have your personal beliefs held more sacred than every other position would make you an irredeemable asshole).
While I may be opposed to religion in the abstract, I wholeheartedly support the right of every individual to hold whatever supernatural beliefs they like, provided these beliefs do not cause harm to others or restrict anyone else’s rights. I find evangelism deeply annoying, but the right to free speech guarantees the freedom to discuss religion just as much as it assures the ability of the nonreligious to disagree.
In essence, believe whatever you like, but no one has the right to force their beliefs on anyone else. If you do feel obligated to spread your message of faith, you have no right to take offense when someone publicly contradicts you.