Imam Ahmad ibn Hambal was a shining star at a time when the sky was almost empty. There were Caliphs at his time who were obsessed with Greek paganistic and materialistic materials. If these Greek sciences had stopped at material science, it would have been beneficial for Muslims; the matter of fact is that the metaphysical materials related to theology have crept in and corrupted the Islamic idea of Allah.
For example, the debate manifested itself on the nature of the Quran: is it a creation of Allah, or is it the Words of Allah and hence an attribute of speaking by Allah? Imam Ahmad resorted to the prime source of the Quran and authentic hadith and gave the verdict that the Quran is the words of Allah and is not among the creations of Allah. The Quran, when written in book form with ink on papers cut from trees, then these are material objects and creations of Allah. However, the contents and words within the Quran are not creations of Allah. The opposite camp, who were influenced by Greek theology and was supported by the Caliph, said that the Quran is a creation of Allah and is not an attribute of Allah.
Today, you and I would naively say, “Why is the big fuss about such a small matter.” Why be so dogmatic about such a small issue? The Ummah should be worried about much bigger issues, and the list goes on and on.
Imam Ahmad was the gatekeeper at his time on a matter of principle that dictates what should be the source of Islamic theology. Giving a verdict about Allah must come from Quran or the authentic Sunnah because Allah is unseen, and no material science can run lab experiments and give a verdict about Allah. The knowledge about Allah must come from an authentic source beyond human material sciences. The Greek sources do not merit being an authentic source to give verdicts about Allah or any matters that are beyond the physical and material sciences.
If Imam Ahman opened the door to contaminate the pure sources of Islamic theology - even by an inch - then over time, the door would get wider and wider, and every tom-dick-and-harry will start giving verdicts about serious theological matters.
The situation at the time of Imam Ahmad was doubly difficult because of the support of the legitimate Islamic Caliph to the ideology of the Greek camp. Not only that the Caliph supported the opposite camp, he employed his army and authority to defend the wrong camp and persecute the Ulema, who refused to bulge. Many Ulema went underground, and many more concealed the truth in their heart and supported the Greek camp out of fear.
Imam Ahmad had the option to conceal the truth and believe it firmly in the heart but agree overtly to the pressure of the Caliph, but he knew that if he did so, no one else would remain to raise the flag of the Quran and Sunnah. He was the single doorkeeper by whom Allah wanted to save Islamic theology from the tsunami of Greek invasion.
He was put to tremendous torture but never deviated a single inch from the right path. Like always, there were BAD ulema who added fuel to the rage of the Caliph and used the language of the Quran and Sunnah to defend Greek theology. These ulema exist everywhere and every time and change color like a lizard depending on what the Caliph wants, and every time they would twist some Quran and Hadith materials to serve the Caliph. They can - with a stroke of a pen - give fatwa making music halal, wine halal, usury halal, etc.
Imam Ahmad studied history well and took lessons from what the Jews and Christians did to their Scipture. Their scripture (i.e., Torah and Gospel) were pure words of Allah. However, the Jews and Christians started to get influenced by other sources and corrupted the Book. If Imam Ahmad allowed the Greek philosophies to enter from one single angel, it would be only a matter of time before they would use that angle to make it legitimate for Greek philosophies to corrupt Islam from other angels.
It is the mercy of Allah that He keeps the Quran intact and that He has arranged to send in every era Ulema who expose those who want to corrupt the pure sources of Islam. As Islam spread physically through military power, it kept winning in the intellectual battlefield. All it requires is patience and seeking the help of Allah.
Here is a nice story. When Imam Ahmad was enduring tremendous torture in his prison cell, he was approached by a thief sharing his prison cell. The thief said, “Oh Imam Ahmad, I am staying patient in front of powerful lashes every day for stealing which is Haram. Can’t you stay patient in defending Quran and Sunnah”? This wisdom from a thief further kept Imam Ahmad on the straight path.
Time passed, those BAD ulema died, and those Caliphs died, and what remained was Imam Ahamd, who earnestly won the title of Imam Ahl Assunnah wal Jamaah—the Imam of the people of Sunnah and Islamic Congregation.