Well, in one sense you’re right. The focus of Christian preaching must always be the Gospel. But which Gospel? The Gospel before Darwin always entailed preaching against public evil, including political evil. Christians were to be salt and light, a testimony to each generation of the high standards of God’s righteousness, and man’s abysmal failure to attain those standards, and thus his urgent need for personal forgiveness and redemption through the blood of the New Covenant shed on the cross by the Son of God, Jesus Christ, on behalf of failed sinners.
After Darwin, and the so-called Enlightenment in general, the intellectual leadership of the churches was besieged with a new and alien worldview, and many responded, even here in America, by simply withdrawing into a safe, pietistic shell, positing a complete dichotomy between spiritual life and public life, a false Gospel that has no prophetic message to sinners at all levels of society, that would probably disapprove of John the Baptist calling out Herod for his adulteries, as a matter of public discourse.
This new half-Gospel opened the door to many significant advances by the essentially atheistic progressive movement, including among many other things the development of tax law which tends to reinforce that false dichotomy, and that certainly is bad enough.
But there are even more dire consequences. Pietism also correlates to the rise of subjectivism, a spiritual beachhead won for atheism by the retreat from public discourse of the preaching of the righteousness of God as a universal standard of moral obligation. Hardly anything could be done to better insulate lost souls from the message they need to hear, than for the preacher to refuse to take on the gross sins of political leadership, no matter the party, as a clear demonstration that every soul, from low to high, must answer to a Holy God.
And there is another dimension, peculiar to the American experience. We do not live under a Roman dictatorship, in which ordinary persons had no particular obligation, other than to obey the law and the whims of their masters, and on occasion to worship Caesar as their god. No, we live in a Republic, in which, at least theoretically, each and every citizen has a duty to participate, at a minimum, through informed voting. Unlike the ancient tyrannies, we hire our public servants. It is our duty before God to be responsible employers, and so we have no choice but to ensure we do not hire those who would avowedly push people into open rebellion against God. A faithful pastor ought not fail to inform his flock of these obligations and how best to fulfill them.
It is our lack of doing this due diligence in which we have failed those who entrusted this Republic to our care, and there will be consequences. Hiding from those consequences by a fresh retreat to pietism will not work, nor will lost souls benefit from a Gospel message that has been watered down to appease an insecure Caesar.