Srimad Bhagavatam, Mahatmyam, Chapter One Sloka 17: Ragnyomoksham.
Translation: Brahma was surprised how a King (Parikshit) destined to die of serpent bite could get moksha. In Brahmaloka, he immediately ordered for a weighing scale and put all the elements of moksha on one pan and Srimad Bhagavatham on the other for weighing.
Interpretation: After the Devas departed empty-handed, Brahma was surprised. He wondered how King Parikshit, who was cursed to die due to serpent bite, could get Moksha by just hearing Srimad Bhagavatam. So he tried to compare Moksha and Srimad Bhagawatham through an experiment. But Brahma knew the truth; he was only trying to explain to ordinary mortals like us.
Srimad Bhagawatham, Mahatmyam, Chapter One Sloka 18: Laghunya..
Translation: Brahma and the rishis were surprised to see that Srimad Bhagavatham outweighed all the elements needed of moksha (liberation of the soul).
Interpretation: What makes Srimad Bhagawatham so special is that the holy Purana is on the Supreme, or Maha Vishnu. It is packed with pure bhakti. Hence it outweighed all elements of Moksha.
Exponents of Srimad Bhagawatham have said that they would indeed opt for a cycle of birth and death and not liberation provided they get an opportunity to read or hear the Holy text.
But even Brahma having a doubt is surprising. However, this element of surprise is meant only to convey a message to ordinary people like us. We should remember that Brahma was the first noble soul to hear the Bhagawatham from the Almighty himself. All that the Supreme Brahman had to do was to narrate the Bhagawatham in just four crisp slokas. It was like zipping the entire Srimad Bhagavatam to one small folder. But that was enough for Brahma to understand. He could unzip the file and realise the absolute truth about Brahman.
But for mortals like this, we need hand-holding through explanations, stories and examples. Even then, we find it difficult to retain the essence of the stories.
The Chatur Sloki Bhagawatham is well described by Sri Bhaktivinoda Thakura as follows:
(1) O Brahma! I, the Personality of Godhead, was existing before creation when there was nothing but Myself. There was no material nature, the cause of this creation. That which you see now is also I, the Personality of Godhead, and after annihilation what remains will also be I, the Personality of Godhead.
(2) O Brahma! That which appears to be of value, but has no relation to Me, has no reality. Know that it is My illusory energy, a reflection, which appears in the darkness.
(3) O Brahma! Please know that the universal elements enter into the cosmos; similarly, I Myself also exist within everything created, and at the same time I am outside of everything.
(4) A person searching after the Supreme Absolute Truth, the Personality of Godhead, must certainly search in all circumstances, in all space and time, and both directly and indirectly.
Many of us may not have understood anything. Do not despair. This was a dialogue between the Supreme and Brahma. Imagine if Isaac Newton and Albert Einstein were to discuss about dimensions of gravity, event horizon, black holes, the bending of time in space etc. We would be clueless. But ask Newton of Einstein to explain what was discussed to ordinary folks like us. That too would be difficult. This is why the Supreme Being entrusted Shri Sukha Muni to explain Srimad Bhagawatham in understandable terms.
In essence, Brahma had no doubt on the importance of Srimad Bhagavatam, but to show the importance of the Purana, he tried to compare the Holy text to Moksha through an experiment or example. And this we all could understand because that is what we do in our daily lives. We tend to compare people, things and objects.
For example, we say “you look bright like a sun; or You are as beautiful as a full moon” etc