Every once in a while I restate the Shahadah and try to do it sincerely. There are certain sins that are kufr (disbelief) and to engage in them, even absentmindedly, will for all intents and purposes, make you a kafir (and if you were to then die, you'd die as a kafir... which is bad). Especially the ones mocking any aspect of the deen or the Prophet(saw)'s Sunnah. It's so easy to get carried away when joking or messing around anyway. You think you've still got the right thing at heart, but I guess everyone thinks they do, so actions (and speech as an action) count.
Though this doesn't apply to me, getting your nikah re-done goes along with that. If a Muslim slips into kufr (and is a kafir in Allah's view), then an Islamic nikah is automatically dissolved so they'd be racking up sins for zina and their children will be illegitimate (and Islamically, there's a consequence for that too in their lives).
Some major books of fiqh on the four Sunni Imams actually recommended that Muftis and other Ulema have mass re-statements of Shahadah and Nikah among the people routinely. o_0
It's not just from the outright kufr sins (mocking or belittling as disbelief or outright disbelief), but for instance, repeatedly and deliberately sinning, or neglecting a Fardh repeatedly, even if you keep it hidden and don't say anything and still have Islam 'at heart'... Allah knows you and your intention better than you. That neglect could very well be an act of kufr. Especially say neglecting the prayers repeatedly for no reason which is one of the biggest fardh duties. Such a gray area so I think it's a good idea. Especially since, as far as worldly affairs go (i.e, law), Islam's a lot more forgiving than other belief systems... for instance, Freud's awesome quote 'He who does not live according to his belief does not believe' should be common sense but even that basic principle is applied liberally in Islam to the point where you just gotta wonder what Allah Himself would really think, regardless of what we've turned our religion into here.
There's other kinds of behaviour which would qualify emphatically as mocking or disbelieving that we think are innocent. If a family pressures a son to shave his beard for marriage, that could be belittling a strong Sunnah of the Prophet(saw) which could make it an act of Kufr. They ought to renew their vows of Shahadah before they take any vows of Nikah because the latter can only count for Muslims. To not wear Hijab or to not grow a beard or to look at forbidden things are all sins (that pretty much everyone I or you know engages in), but not acts of Kufr (especially if the person knows, acknowledges, and believes it is indeed a sin and is remorseful for it, but with repeated acts that have become a lifestyle, God knows). But if someone disagrees with the concept of Hijab, doesn't believe in it, and belittles it or mocks it (which is what usually happens when they don't believe in something and are pressured by their family and community into it), that could also be an act of Kufr. Same for someone who hates the beard, or openly thinks it's retarded to not look at forbidden members of the opposite sex all they want (i.e, no conscience about the activity)... all are acts of Kufr. You can definitively say something is an act of Kufr but what you don't know is if that person has become a Kafir or not. At that point, you gotta stop minding other people's business and wonder about yourself and your own Iman.
In my view, MOST Muslims are safe from most of these kinds of situations of disbelief, but to be 100% sure is another thing. I'm not even that sure about myself.
I guess what it comes down to is that I'm not Muslim just because I think I am or if I feel I am. Maybe we have a tendency to get lax and take forgiveness for granted because we believe in Allah and His Prophet(saw). But belief is not measured that way. There are so many emotions and other feelings we can experience that are pretty much just bullshit, like lust disguised as love for one thing. Why can't "faith" be one of them? Perhaps our 'faith' is just a caricature, the side effect of some psychological disorder. I used to always tell myself I'm Muslim because I think I am and I feel I am. But that can't be the case. I'm Muslim if I'm Muslim, if Allah accepts me as a Muslim. That's what the Qur'an, Hadith, Sunnah, Sahaba, Tabi'in, Ulema, Islamic history, etc are all for, so Allah can tell us what that is.
People should also be careful when joking. Just trying to consciously pay attention to your words is enough to avoid a bad situation. For instance, one might lightheartedly joke about a friend with a beard 'you terrorist'... and that joke's premise is rooted in mocking the non-Muslims' association of beards with Muslims and Muslims with terrorism. That isn't Kufr at all. That could even be called desireable. But in the process of so joking, every once in a while, it's possible that a feeling of disgust or something like that enters the heart when making that joke about someone with a beard... meaning a part of you becomes serious and is no longer joking. Obviously more likely to happen if the one joking doesn't keep a beard because there's the issue of why don't they keep the beard, and if that why comes out in the form of a negative feeling in the heart when 'joking' about the beard on someone else, it will become an act of Kufr right then and there (since it's not really a joke at heart). And who knows if that act takes you out of Islam. But if you just consciously check yourself whenever approaching anything regarding religion, that usually nips any negative intentions in the bud (so in the above case, the person remembers the beard is Sunnah so when the joke comes out, it's once again mocking the non-Muslims, not the Sunnah and instead of disgust, remorse is felt for not following the Sunnah themselves even if they do dislike the beard which in and of itself is nothing).
You've always known that the world today is extraordinarily dangerous for Islam and Iman, when you take into account these things, then that becomes clearly visible and not easily forgotten. There's a Hadith that says near the Day of Judgement, for a Muslim to hold onto their Iman would be like holding onto a ball of fire. I know that's 100% true for many people already. They can say the Shahadah sincerely, believe, and unwittingly within hours fall out of Islam again. If you're in a similar situation, just try to stick to the basic fardh obligations and repeat the Shahadah with sincerity whenever you remember. So you'll be Muslim at least five times a day. This kind of sounds ridiculous, but ya... its not a joke.
Slightly related so I might as well tack it on: The worst 'loser' (search the Qur'an for that word) in the 'Eyes' of Allah is that person who's astray but believes they're on the right path. Their 'Aqeedah or Iman is there but it's not the Iman of Rasulullah(saw) so they've gone on the path to Kufr and out of Islam... and they don't know. In fact, Shaitan has them convinced they are on the right and they try to get others to follow them... as per the Hadith that says there will be 'Muslims' inviting other Muslims to Hell in this time.
At this point you can pretty much insert every Hadith about the End Times, but you get the idea.