Is the Calamity of One Who Doesn’t Pray Greater than that of Iblis?
Question

Is it valid to say that the calamity of one who does not pray is greater than the calamity that befell Iblis, because Iblis refused to prostrate to Adam, and the one who does not pray has refused to prostrate to the Lord of Adam?
Praise be to Allah.The meaning of this phrase is not correct for several reasons:

When Iblis disobeyed by not prostrating to Adam , he was not disobeying the command of Adam (peace be upon him); rather he was disobeying the command of Allah, may He be exalted. Thus he was no less an evildoer than the one who disobeys the command of Allah, may He be exalted, to pray . Hence he became a disbeliever (kafir), because he did not prostrate.

Allah, may He be exalted, says (interpretation of the meaning): 
{And [mention] when We said to the angels, “Prostrate before Adam”; so they prostrated, except for Iblis. He refused and was arrogant and became of the disbelievers.} [al-Baqarah 2:34]
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said:
“Prostrating to anyone other than Allah, if it is done in response to the command of Allah, is an act of worship, because Allah, may He be exalted, has the right to rule however He wills. Hence when Iblis refused to do that, he became one of the disbelievers.” (Tafsir Surat al-Baqarah, p. 126)

When Iblis disobeyed Allah, may He be exalted, that was accompanied by objection to the command of Allah, may He be exalted, and being too arrogant to obey Him. Undoubtedly this is worse and is more contrary to the veneration of Allah, may He be exalted, which is the foundation of faith.

As Allah, may He be exalted, says (interpretation of the meaning):
{[Allah] said, “What prevented you from prostrating when I commanded you?” [Satan] said, “I am better than him. You created me from fire and created him from clay.” [Allah] said, “Descend from Paradise, for it is not for you to be arrogant therein. So get out; indeed, you are of the debased”.} [al-A'raf 7:12-13]

The disobedience of Iblis is worse, because the proof against him was greater. He saw of the might of Allah, may He be exalted, and the greatness of His authority that which the one who does not pray has not seen.

Ibn al-Qayyim (may Allah have mercy on him) said:
“The severity of disbelief (kufr ) which leads to severity of punishment is due to three reasons…
The second of which is: deliberately going to extremes in stubbornness and misguidance despite being convinced, such as the disbelief of those who accepted in their hearts that the Messenger was true because of what they saw of the signs of his truthfulness, but they disbelieved out of stubbornness and resentment, such as the people of Thamud, the people of Pharaoh and the Jews who recognized the Messenger as they recognised their own sons, and the disbelief of Abu Jahl, Umayyah ibn Abi’s-Salt and their ilk.” (Tariq al-Hijratayn, 2/895)
It is clear from what we have mentioned in the answer that this is not to underestimate the issue of not praying, for the evidence of the Quran and Sunnah, and the consensus of the Sahabah, indicate that the one who does not pray is a disbeliever , and that he has no share of Islam. However, disbelief (kufr) is of varying levels, some of which are more serious than others, just as some types of evil are less grievous than others.
And Allah knows best.