"My sense is that we do not realise what a calamity is happening around us," Piper, pastor of Bethlehem Baptist Church in Minneapolis, wrote in a commentary on Thursday. "Christians, more clearly than others, can see the tidal wave of pain that is on the way. Sin carries in it its own misery."
It's been nearly a week now since marriage for gay and lesbian couples was legalised in New York and since hundreds of thousands of Americans celebrated homosexuality with gay pride parades, not only in New York but also in Piper's home state of Minnesota.
Homosexuality and its celebration are nothing new, the Reformed pastor clarified.
"[Homosexuality] has been here since we were all broken in the fall of man," he wrote. "What's new is not even the celebration of homosexual sin. Homosexual behaviour has been exploited, and revelled in, and celebrated in art, for millennia.
"What's new," he underscored, "is normalisation and institutionalisation. This is the new calamity."
America, and the rest of the world, is moving toward the institutionalisation of homosexuality, the 65-year-old pastor lamented.
Yet the Bible makes it clear that homosexual behaviour is sin, he said.
"Alongside its clearest explanation of the sin of homosexual intercourse (Romans 1:24-27) stands the indictment of the celebration of it," he said.
"Though people know intuitively that homosexual acts (along with gossip, slander, insolence, haughtiness, boasting, faithlessness, heartlessness, ruthlessness) are sin, 'they not only do them but give approval to those who practice them' (Romans 1:29-32).'I tell you even with tears, that many glory in their shame' (Philippians 3:18–19)."
For the first time since it began tracking the issue of same-sex marriage in 1996, a Gallup poll last month found that a majority of Americans (53 per cent) believe marriages between same-sex couples should be recognised by law as valid.
Moreover, 56 per cent of Americans say gay or lesbian relations is morally acceptable, another Gallup poll found in May. Only 39 per cent perceive homosexual relations as morally wrong.
Piper stressed that his purpose for writing on the controversial issue is "not to mount a political counter-assault".
He doesn't believe that is the calling of the church.
Rather, Piper expressed his desire to "help the church feel the sorrow of these days. And the magnitude of the assault on God and his image in man."
He didn't pin the sin of sexual immorality on homosexuals alone, however. Heterosexuals are just as guilty.
Piper emphasised that Jesus died for both heterosexual and homosexual sinners so that they might be saved. Jesus, he stressed, offers "astonishing mercy".
But rather than embracing that salvation, thousands celebrated sin last weekend, he lamented.
"Christians know what is coming, not only because we see it in the Bible, but because we have tasted the sorrowful fruit of our own sins. We do not escape the truth that we reap what we sow. Our marriages, our children, our churches, our institutions – they are all troubled because of our sins," he wrote.
"The difference is: We weep over our sins. We don't celebrate them. We turn to Jesus for forgiveness and help. We cry to Jesus, 'who delivers us from the wrath to come (1 Thessalonians 1:10).'"
"And in our best moments, we weep for the world."
The win in New York for gay rights activists is expected to propel the gay marriage movement forward. Already, they are working to push similar legislation in Maine and to defeat a measure amending the constitution to define marriage as between a man and a woman in Minnesota.
Amid the movement to redefine marriage, Piper made it clear that Jesus created sexuality and "has a clear will for how it is to be experienced in holiness and joy".
"His will is that a man might leave his father and mother and cleave to his wife, and that the two become one flesh (Mark 10:6-9). In this union, sexuality finds its God-appointed meaning, whether in personal-physical unification, symbolic representation, sensual jubilation, or fruitful procreation."
Nevertheless, there are no signs of the gay marriage movement slowing down. With that, Piper left Christians with this concluding note:
"This is what I am writing for. Not political action, but love for the name of God and compassion for the city of destruction. 'My eyes shed streams of tears, because people do not keep your law.' (Psalm 119:136)."