While Lucifer and Michael were fighting Jack had been absorbing the energy that came off from them, including the energies released from the two archangels deaths. When he absorbed enough power he became unstoppable, immune to even God's attempts to smite him.
In The End, Jack Became A “Hands-Off” Deity
Jack sucked up all their power and he was the one to kill Chuck, and take all his power too. He was more powerful than God and Amara together. He put the whole world to rights but didn't bring back all the people that had died because of his plan.
After the second death of Lucifer and the betrayal and death of Michael, Jack absorbs God's power and becomes the new God, the effort to turn him into a cosmic bomb having also made Jack a power vacuum that sucked up all of the power around him, including from the fights with Michael, Lucifer and Chuck.
Using their fake "spell," God is lured into beating up the Winchesters, where he expends more of his power for Jack to secretly feed upon. After amassing enough energy, Jack overwhelms his grandfather and takes the power of the Almighty for himself.
Beings. Primordial Beings: Jack is more powerful than any of the archangels, though less powerful than God or the Darkness.
Jack Kline: Being the Nephilim son of Lucifer and far more powerful than Archangels, Jack's power was growing to the point where God feared that he would become more powerful than even himself to the point where he did not hesitate to admit this fear to Sam about his potential and his fear of Jack even trumped his ...
Despite being the youngest member of the family, Jack-Jack has the widest array of superhuman abilities, most of which are centered around shapeshifting. Edna Mode classified Jack-Jack as a polymorph, likening his many powers to him being a baby and thus having limitless potential.
In Inherit the Earth, Jack absorbs all the power of God and The Darkness, who was merged with God and becomes the new God. Thus, Jack now has his and every power that God and Amara once possessed.
At the end, Jack coughs up blood in front of Dean before suddenly collapsing, bleeding from his nose and mouth. Rowena determines that Jack's condition is a result of his human and angel sides attacking each other without his grace maintaining a balance.
With Jack stepping into the role of God, the world went back to normal — which is something, considering how far from normal we all are in 2020 — and each and every person Chuck erased came back to their old selves. But hoo golly has Season 15 done a good job of tying up loose ends!
According to Billie, Jack's status as a nephilim means he harbors the potential strength to take down God, but he'll need a considerable power-up to reach that level. This is why Jack is found eating the angel hearts - presumably to inherit the grace of his victims.
Billie resurrected Jack and sent him to Earth to feed on Grigori hearts in an effort to make him stronger so that eventually he'll be able to fight Chuck. He would have checked in with Cas and the Winchesters sooner, but he needed to lay low so that Chuck, who fears him, wouldn't find him and kill him again.
Chuck Shurley is one such character who started out as a prophet of the Lord, guest-starring in some of the earlier seasons, and then made a sudden comeback over the last few years as God.
Trimurti is considered to be the most powerful god as he is a combination of Brahma [The Creator], Vishnu [The Preserver] & Shiva [The Destroyer].
Higher Beings - According to Billie, the Empty is not as powerful as God or Amara.
Indra also called Śakra, the supreme god, is the first of the 33, followed by Agni.
Jack later explains that he has been eating Grigori hearts as part of Billie's plan for him to become strong enough to kill God Himself.
Losing his grace has caused an impact on his mortal body
On the other hand, the second explanation for Jack's sickness, which may be more likely, is that Jack is feeling the physical effects of losing his grace. As I've mentioned, we've never seen the outcomes of grace being extracted from a Nephilim before.
After a soulless Jack kills Nick for trying to resurrect Lucifer, he accidentally kills Mary in a brief fit of rage. Mary is reunited with her husband in Heaven and is now at peace in a shared Heaven with John.
AT FULL POWER, HE RIVALS ARCHANGELS
Since Jack is the son of Lucifer, this means that he will not only be stronger than his father once he reaches full power, but he will also most likely be stronger than all of the archangels.
After Jack's human soul fused with his Nephilim grace, he exploded into a metaphysical supernova, essentially making him capable of absorbing celestial energy from everything around him. He eventually uses this to steal Chuck's power, rendering him human as Jack becomes the new God.
Jack's eyes light up as he uses his soul to power Enochian magic to stop Michael. Following the death of Michael, the Winchesters and Castiel are left worried about how much of Jack's soul he burned away. Castiel tells Dean that he doesn't think its all gone, that there's some left, but he's not sure how much.
As CinemaBlend reported, Jack-Jack appears to have an unlimited list of abilities, including laser eyes and combustion inducement, which are two abilities seen in the “Incredibles 2” trailers. “Incredibles 2” director and writer Brad Bird said Jack-Jack doesn't have unlimited gifts, though.
Jack-Jack is just a baby, but he is likely the most powerful superhuman in the world of The Incredibles. He had no powers in the first movie until the end, when he was able to turn into some kind of demon to escape the clutches of Syndrome. Then, in Incredibles 2, he showed how powerful he really was.