The truth is that the Nazi (National Socialists) of Germany were not Christian, even though they may have claimed to be. The truth is that they persecuted and executed genuine Christians like Dietrich Bonhoeffer. Here is a summary from Wikipedia: Dietrich Bonhoeffer (German: [ˈdiːtʁɪç ˈboːnhœfɐ]; 4 February 1906 – 9 April 1945) was a German Lutheran pastor, theologian, anti-Nazi dissident, and key founding member of the Confessing Church. His writings on Christianity's role in the secular world have become widely influential, and his book The Cost of Discipleship became a modern classic.
Apart from his theological writings, Bonhoeffer was known for his staunch resistance to the Nazi dictatorship, including vocal opposition to Hitler's euthanasia program and genocidal persecution of the Jews. He was arrested in April 1943 by the Gestapoand imprisoned at Tegel prison for one and a half years. Later he was transferred to a Nazi concentration camp. After being allegedly associated with the plot to assassinate Adolf Hitler, he was briefly tried, along with other accused plotters, including former members of the Abwehr (the German Military Intelligence Office), and then executed by hanging on 9 April 1945 as the Nazi regime collapsed, just two weeks before Allied forces liberated the camp and three weeks before Hitler's suicide. https://en.wikipedia.org/wiki/Dietrich_Bonhoeffer
Saying, "I am a Christian," does not make us Christian any more than saying, "I am a duck," would make me a duck. The Bible makes is very clear that we are not a Christian unless Jesus Christ recognizes us a His followers. The guy on Facebook thought that just because Hitler (may have claimed) to be Christian, that made it true but he was mistaken. Here is what the Bible says.
Jesus said, “Beware of false prophets, who come to you in sheep’s clothing, but inwardly they are ravenous wolves. You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles? Even so, every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Therefore by their fruits you will know them. “Not everyone who says to Me, ‘Lord, Lord,’ shall enter the kingdom of heaven, but he who does the will of My Father in heaven. Many will say to Me in that day, ‘Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name?’ And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness!’ Matthew 7:15-23 NKJV https://www.biblegateway.com/passage/?search=Matthew+7%3A15-23&version=NKJV
We can know that Adolf Hitler and the Nazis were not Christian, "by their fruits." On the other hand, Pastor Dietrich Bonnhoeffer's fruits indicate that he was a genuine follower of Jesus Christ, a real Christian.