Praise be to Allah.

Firstly: 

We should note that the Prophet (peace and blessings of Allaah be upon him) told us that this ummah would become divided. 

It was narrated from Abu Hurayrah (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The Jews split into seventy-one or seventy-two sects, and the Christians split likewise. My ummah will split into seventy-three sects.” According to a report narrated by Ibn Maajah (3993), from the hadeeth of Anas, “… all of whom will be in the Fire apart from one.” 

Narrated by al-Tirmidhi, 2640. Abu ‘Eesa al-Tirmidhi said: The hadeeth of Abu Hurayrah is a saheeh hasan hadeeth. It was classed as saheeh by al-Albaani in Saheeh Ibn Maajah, 3227. 

Once this is understood, the saved and victorious group is one and it is the group that adheres to the Qur’aan and Sunnah inwardly and outwardly. 

Secondly: 

The one who makes a claim to divine lordship is a worse kaafir than the pre-Islamic mushrik Arabs to whom the Prophet (peace and blessings of Allaah be upon him) was sent. The mushrikoon in the Jaahiliyyah used to believe that Allaah was the Creator and Provider, the Giver of life and death. The evidence for that is the verse in which Allaah says (interpretation of the meaning): 

“And if you (O Muhammad) ask them: ‘Who has created the heavens and the earth,’ they will certainly say: ‘Allaah’”

[Luqmaan 31:25]

Yet despite that Allaah did not say that they were not mushriks and kaafirs, because they denied Tawheed al-‘Uboodiyyah (Oneness of worship), which means worshipping Allaah alone. They used to prostrate to “deities” other than Allaah and offer sacrifices to “deities” other than Allaah; they associated idols in worship with Allaah and they thought that they (the idols) could bring benefit or cause harm instead of Allaah. Hence Allaah tells us that they said, 

“ ‘Has he made the aalihah (gods) (all) into One Ilaah (God — Allaah). Verily, this is a curious thing!’”

[Saad 38:5 – interpretation of the meaning]

And Allaah says (interpretation of the meaning): 

“And those who take Awliyaa’ (protectors, helpers, lords, gods) besides Him (say): ‘We worship them only that they may bring us near to Allaah’”
[al-Zumar 39:3]

 This indicates that the kufr of the mushrik Arabs was less serious than the kufr of those mentioned in the question, because the mushrik Arabs affirmed the Oneness of divine Lordship (Tawheed al-Ruboobiyyah) whereas these people deny it. 

Allaah has refuted in the Qur’aan the specious arguments of those who lay claim to divine lordship. Allaah says (interpretation of the meaning): 

“Had there been therein (in the heavens and the earth) aalihah (gods) besides Allaah, then verily, both would have been ruined”

[al-Anbiya’ 21:22] 

“No son (or offspring) did Allaah beget, nor is there any ilaah (god) along with Him. (If there had been many gods), then each god would have taken away what he had created, and some would have tried to overcome others! Glorified be Allaah above all that they attribute to Him!”

[al-Mu’minoon 23:91]

The story of the arguments that Ibraaheem (peace be upon him) used against those who made claims of divine lordship shows us the best way of arguing against them. Allaah says (interpretation of the meaning): 

“Have you not looked at him who disputed with Ibraaheem (Abraham) about his Lord (Allaah), because Allaah had given him the kingdom? When Ibraaheem (Abraham) said (to him): ‘My Lord (Allaah) is He Who gives life and causes death.’ He said, ‘I give life and cause death.’ Ibraaheem (Abraham) said, ‘Verily, Allaah brings the sun from the east; then bring it you from the west.’ So the disbeliever was utterly defeated. And Allaah guides not the people, who are Zaalimoon (wrongdoers)”

[al-Baqarah 2:258]

 So follow this example and say to the one who makes claims of divine lordship, “If what you are saying is true, then let him bring the sun from the west, or bring the dead back to life, or cause rain to fall from the sky, or bring forth crops from the earth.” 

Fourthly: 

The Prophet (peace and blessings of Allaah be upon him) has told us that the Dajjaal who will emerge at the end of time will claim to be divine. The Prophet (peace and blessings of Allaah be upon him) has told us the sign by which we may know that his claim to be God is false. This sign is that the Dajjaal will be one-eyed, blind in his right eye. 

It was narrated from Ibn ‘Umar (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) stood up to address the people. He praised Allaah as He deserves to be praised, then he mentioned the Dajjaal and said: “I warn you of him, and there is no Prophet who did not warn his people of him. Nooh warned his people, but I am warning you in words that no other Prophet has said to his people. You should know that he (the Dajjaal) is one-eyed and Allaah is not one-eyed.” (narrated by al-Bukhaari, 2892; Muslim, 169). 

This sign will be obvious to everyone who sees it. If this Dajjaal was the Lord Who brought this universe into being with all its beauty, He would be able to remove that ugliness from himself and remove that blindness from his eye. 

Allaah, Who created this universe and created man with his wondrous harmony and perfect shape, must inevitably be absolutely perfect with no shortcomings or imperfections of any sort whatsoever. 

Signs of imperfection are obvious in those who makes claims of divine lordship; they sleep, fall sick, suffer pain, suffer discomfort in heat and cold, need food and drink. Any one of them carries impurities (najaasah) within himself and needs to go to the lavatory one or twice a day! Are such people fit to be called gods?  Glory be to You (O Allaah)! This is a great lie (cf. al-Noor 24:16). 

It is not strange that they make this claim; what is really strange is that they have followers who agree with what they say. 

Fifthly: 

The Muslim must realize that these sects – such as the group mentioned, and the Qadianis, Baha’is and other misguided groups – were only established to fight Islam and lead its people away from it. It does not take much effort to demonstrate that they are misguided and false. The Muslim should beware of them and warn other Muslims against these misguided groups. 

Sixthly: 

With regard to refuting the religion of the Christians, it does not require much effort. All that is needed to refute them is the following words of Ibn al-Qayyim (may Allaah have mercy on him): 

O worshippers of the Messiah, we have a question, to which we want an answer from the one who understands it.

If a god dies because of the actions of people who kill him, what is this god?

Is he pleased with what they did to him? Then they must be lucky for they have earned his pleasure.

If he is displeased with what they did to him, their power has nevertheless overwhelmed his.

Did the universe remain without a god who hears all and answers those who call upon him?

Were the seven heavens left with no god above them when he was buried in the ground?

Was this universe left with no god to look after it when his hands were nailed (to the cross)?

How could this god be forsaken by all his creation when they heard him weep?

How could this wood bear the true god who was tied to it?

How could iron come close to him and penetrate him and wound him?

How could the hands of his enemies reach him when they struck the back of his head?

Was the Messiah brought back to life or is the one who revived him another god?

What a strange grave it is that could contain a god. What is even stranger is that a womb could contain him,

Where he remained for nine months, nourished from blood,

Then he emerged from the vagina as a tiny baby, opening his mouth and seeking the breast,

Eating and drinking, with the inevitable consequences thereof. Is that a god?

Exalted be Allaah above the fabrications of the Christians. He will question them all about the lies they told.

O worshippers of the cross, for what reason is that thing (the cross) venerated?

Rationally speaking, it should be broken and burned.

If god was crucified upon it by force and his hands nailed to it,

Then the thing used for that purpose should be cursed and trampled upon, not kissed when you see it.

How can the Lord of the Worlds be humiliated on it, then you go and worship it?

Then you are the enemies of that god, if you venerate it because the lord of mankind has touched it.

That cross is lost but every time we see something similar it reminds us of that cross.

Then why you do you not venerate the graves, for a grave once contained your lord?

O worshipper of the Messiah, wake up! He has a beginning and he has an end. 

Ighaathat al-Lahfaan, 1/291 

And Allaah knows best.