Belief in Allaah is based on respect and veneration of Him and submission to Him. Hence Allaah criticized the kaafirs and stated that they associated others with Him because they did not make a just estimate of Allaah such as is due to Him, as He says (interpretation of the meaning): 

“They made not a just estimate of Allaah such as is due to Him. And on the Day of Resurrection the whole of the earth will be grasped by His Hand and the heavens will be rolled up in His Right Hand. Glorified be He, and High be He above all that they associate as partners with Him!”

[al-Zumar 39:67]

Allaah, may He be glorified and exalted, is the Almighty, by Whose might the heavens might almost be rent asunder, as He says (interpretation of the meaning): 

“Nearly the heavens might be rent asunder from above them (by His Majesty), and the angels glorify the praises of their Lord, and ask for forgiveness for those on the earth. Verily, Allaah is the Oft‑Forgiving, the Most Merciful”

[al-Shoora 42:5]

Anyone who studies the creation of Allaah will see some of the signs of His greatness. The Prophet (peace and blessings of Allaah be upon him) said, describing the Kursiy (Footstool) and Throne: “The seven heavens, in comparison to the Footstool, are like a ring lying in the wilderness, and the superiority of the Throne to the Footstool is like the superiority of that wilderness to that ring.” Classed as saheeh by al-Albaani in al-Silsilah al-Saheehah (1/223). 

Veneration of Allaah, may He be glorified and exalted, cannot co-exist in one heart with mockery of Him. Hence mocking Allaah, may He be exalted, or His verses or Messengers is kufr, no matter what form it takes, whether it is in earnest or in jest. Allaah says in Soorat al-Tawbah (interpretation of the meaning): 

“The hypocrites fear lest a Soorah (chapter of the Qur’aan) should be revealed about them, showing them what is in their hearts. Say: ‘(Go ahead and) mock! But certainly Allaah will bring to light all that you fear.’

65. If you ask them (about this), they declare: ‘We were only talking idly and joking.’ Say: ‘Was it at Allaah ( عز و جل ), and His Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) and His Messenger (صلى الله عليه وسلم) that you were mocking?’

66. Make no excuse; you disbelieved after you had believed. If We pardon some of you, We will punish others amongst you because they were Mujrimoon (disbelievers, polytheists, sinners, criminals)”

[al-Tawbah 9:64-66]

Shaykh al-Islam Ibn Taymiyah said: This shows that mocking Allaah and His verses and signs is kufr and this verse indicates that the one who belittles the Messenger of Allaah in earnest or in jest is a kaafir. 

Al-Saarim al-Maslool (2/70). 

Al-Qaadi Abu Bakr ibn al-‘Arabi said in Ahkaam al-Qur’aan commenting on this verse: What they – the hypocrites – said must have been either in earnest or in jest, but no matter what it was, it is kufr, because joking about kufr is also kufr, and there is no difference of opinion concerning this among the ummah. End quote. 

The great scholar al-Sa’di said: Mocking Allaah and His Messenger is kufr which puts one beyond the pale of Islam, because the religion is based on veneration of Allaah and of His religion and His Messengers, and mocking any of that is contrary to this principle and is diametrically opposed to it. 

What this man said, in which he showed disrespect towards Allaah – exalted be He far above what the wrongdoers say – and spoke as if he regarded Allaah as equal in status to a human being and thought that he could address Him as such, is kufr, as anyone who has the slightest knowledge of the religion of Allaah would realize, and no one would do that but one who is extremely ignorant or a man whose heart has no knowledge of Allaah and no respect for Him. 

Then he added to his mockery and said “I must tell Allaah.” Does Allaah, may He be exalted, need to be told anything by an ignorant wrongdoer?! 

“Truly, nothing is hidden from Allaah, in the earth or in the heaven”

[Aal ‘Imraan 3:5]

This wretch has to renew his faith and enter Islam again, and repent to Allaah from this blatant kufr. For the rest of his life he must do a lot of good and righteous deeds, as much as he can, in the hope that Allaah may pardon him and forgive him for his ignorance and transgression. 

And Allaah knows best.