All good points my friend. Nigerian Christians should participate in building their nation, for the reasons you give. I've had this conversation with a few Jehovah's Witnesses (when they come to my door I do the preaching LOL!) and said many of the same things.
But here's the other side of that coin, and one valid reason Christians might want to stay out of politics, or join the Libertarian Party if there is one. Political parties tend to see themselves as an instrument in creating a moral society where the laws are hoped to form a more moral people. Many Christians who understand the Gospel as inward transformation so that people won't want to do immoral acts. The goal of the Christian faith is to get to a place where no laws are even needed. People just do the right thing intuitively. Because Christ has been formed within.
Christians who understand this have something in common with members of Libertarian Parties. Both realize that laws are not very useful for forming a virtuous society, and attempts to do so only amount to government interference in people's personal lives.
So what many Christians and/or Libertarians realize is that laws are really only good for one thing: preventing people from doing harm to others. And it doesn't take religious people to do that. In fact, religious people haven't had a very good track record with that because they tend to overstep their boundaries and force their will and morality on others when the others are not harming anyone.
Thanks gorgeous contribution