It won't last, because he's not socially conservative. He's either said nothing or given very mixed answers on social issues, eventually the socially conservative voters will shift to Cruz, Huckabee, or maybe Walker, Rubio, or Carson. Someone who actually agrees with them on issues.
You're implying white evangelicals are a thinking group of voters. These are the same people that gave their money to Peter Popoff. They have no principles. They'll have faith in Donald Trump.
Life must be really easy for you, huh? When you can simply write off a whole group of decent Americans simply because you disagree with them.
White Evangelicals have been demonized by the World and dismissed as lunatics and fanatics routinely. Being one, I understand this; Jesus, Himself said to his disciples: "If the World hates you, remember that it hated Me, first." Being written off by folks isn't (and shouldn't) be too much of a surprise for believers. But to say such folks have "no principles" is unfair to the point of slander. If I were to say this about the BLM folks, I'd be pilloried as an insensitive racist. (Of course, if I advocated putting on trial every police officer who used deadly force, even when there was no probably cause to indicate that a crime had been committed, I'd be labeled a "compassionate progressive", so . . .)
White Evangelicals have faith in GOD, not in Donald Trump. That the faith of white Evangelicals leads many of them to, pretty much, reject the Democratic Party out of hand should not be surprising. The Democratic Party has made certain positions on Biblically relevant issues (abortion, SSM) litmus tests. Even worse, the Democratic Party as a whole has taken a rhetorical posture of hostility toward Evangelicals to the point where many have concluded that the Democratic Party is, indeed, anti-Christianity, and views Christianity as evil (unless it's from black churches who sponsor Souls to the Polls efforts).
White Evangelicals are concerned about the state of our nation, and it's role in the World. They are very much concerned with a declining America. They believe that America is both a GREAT nation and a GOOD nation, and they believe that the world is better off BECAUSE OF AMERICA. This is hardly an incoherent belief, and they are concerned with the self-flagellation many folks wish to subject America to; they see such a posture as defeatist and dangerous. And they see the decline in morality in America as a factor that drains us as a nation. They see the decline in the institution of the traditional family as an event that is undermining the viability of America as a nation, and they are not wrong about this in the least. And they have seen any number of Republican politicians give them lip service on these issues while campaigning, only to do nothing about them once elected.
Trump, of all people, is addressing THESE concerns. Evangelicals support Trump not because he's their religious example, but because he addresses their specific concerns about the overall condition of America as a nation, and not just some list of issues they think are important. Trump's not promising them Moral Utopia, but he IS promising them a more respected and more effective America, and these are promises that a President CAN deliver on. And Trump is a person that IS respected and HAS accomplished things that actually suggest that he can improve the standing of America in the World and reduce the perception (and the actuality, for that matter) of American Decline. Those aren't unimportant issues, and Trump is probably MORE likely to address them than any other of the mediocrities running for President in both parties.