The secular left has suddenly embraced Jesus's supposed socialism to condemn the Republican tax plan. He's a convenient savior for them. They care about his tax policy, but choose to ignore all of scripture on abortion, marriage, homosexuality, and God creating us male and female. The reality is that the Bible's view of government aid to the poor is more nuanced than the left would like. To the extent they think the government is ordained by God to take care of the poor they have substituted government for God and think they can bring about heaven on earth.
God commands that we take care of the widows, orphans, poor, and refugees. But it is a command to us individually, not to a government. To the extent government reflects our values, government should do those things as well. To the extent government reflects our values, it should also support traditional marriage and a culture of life that does not fund Planned Parenthood. The left, however, ignores that.
So yes, a government that reflects our Godly values should take care of the poor. But the left has been telling us for years that we need a separation of church and state and Christians should note impose their morality through legislation. Only now they seem to want a Christ like tax policy (which, in reality, would probably be a flat 10% tax). It's because they don't really want a Christ like tax policy, they just want to beat up conservatives.
I'm a conservative, not a libertarian, because I believe in a government social safety net for those unable to help themselves through no fault of their own. But the left reads more into scriptures' commands than are there. There is no tax rate that is or is not sinful. The fact is by letting people keep more of their of their money, people are able to do more to help the poor themselves. Again, it is an individual obligation. You cannot offload this individual obligation to the government. So if you are outraged that you are going to start seeing more money in your paycheck you should give that money to the poor. This isn't hard. Actual, practicing Christians give more to charity than liberals because Christians understand this and preening liberals do not. Liberals want to raise taxes and let government take care of the poor. Christians know we cannot abdicate our responsibility to government.
One last point here that needs to be noted. Many of the leftwing "Christian" theologians who are condemning the GOP for lowering marginal tax rates as if it was a sin are aligning themselves with secular, atheists in Hollywood, the Planned Parenthood culture, etc. These people hate the God of these supposed Christians. That should tell them something, but it won't. It should, however, tell you and me a lot when they come sermonizing. They are perfectly happy to echo the world on taxes and never echo Christ on life, marriage, or values.