Denying what has been proven by the Qur'an and the Sunnah (acts, sayings or approvals of the Prophet) and what has been reported in the Hadiths Mutawatir (Hadiths reported by a significant number of narrators throughout the chain of narration, whose agreement upon a lie is impossible) (Part No. 7; Page No. 176) is considered Kufr (disbelief). Whoever denies the fact that asking Allah for rain (through Salat-ul-Istisq' (prayer for rain)) when there is drought is a reason for making rain fall is actually denying the Sahih (authentic) Hadiths that urge Muslims to resort to Allah and seek relief from Him. This also implies disbelief in the Ayahs (Qur'anic verses) that encourage Muslims to turn to Allah in times of adversity as Allah (Exalted be He) says:
"I said (to them): ‘Ask forgiveness from your Lord, verily, He is Oft-Forgiving;
‘He will send rain to you in abundance,
Denying this and having doubts about it makes one's Tawhid (belief in the Oneness of Allah) defective. Moreover, having such a belief and denying the Ayahs and Hadiths reported in this respect is Kufr that takes a Muslim outside the pale of Islam. Hence, the one who claims this must sincerely repent.As for what is mentioned in the question about Kafirs and how they receive a lot of rain despite their Kufr and excessive sins, one must not be deceived by this for there is no proof that Allah loves them or is pleased with them. Rather, Allah may be gradually leading them to their own destruction. Allah gives oppressors time and bestows blessings on them generously and then seize them with punishment after that. Allah (Glorified and Exalted be He) says:
Let not the free disposal (and affluence) of the disbelievers throughout the land deceive you.
A brief enjoyment; then, their ultimate abode is Hell; and worst indeed is that place for rest.
Allah (Exalted be He) says:
...while those who disbelieve enjoy themselves and eat as cattle eat; and the Fire will be their abode.
You received your good things in the life of the world, and you took your pleasure therein. Now this Day you shall be recompensed with a torment of humiliation,
(Part No. 7; Page No. 177) Also, Allah (Exalted be He) says:
...until when the earth is clad in its adornments and is beautified, and its people think that they have all the powers of disposal over it, Our Command reaches it by night or by day and We make it like a clean-mown harvest, as if it had not flourished yesterday!
So, when they forgot (the warning) with which they had been reminded, We opened for them the gates of every (pleasant) thing, until in the midst of their enjoyment in that which they were given, all of a sudden, We took them (in punishment), and lo! They were plunged into destruction with deep regrets and sorrows.
So the root of the people who did wrong was cut off. And all the praises and thanks are to Allâh, the Lord of the ‘Alamîn (mankind, jinn, and all that exists).
.As for the poverty, adversity, scarcity of rain, loss of wealth, lives and crops with which Allah tries his believing worshippers; it is a way to test them and make them more attached to Allah and increase their hope in Him. Whenever they are afflicted with any of these trials, they realize that it is from Allah, hence, they return to Him, implore Him and seek refuge with Him. In this way, their reliance on Allah is reinforced and their faith in Him is strengthened. Allah (Glorified and Exalted be He) says:
And certainly, We shall test you with something of fear, hunger, loss of wealth, lives and fruits, but give glad tidings to As-Sâbirûn (the patient).
Allah (Glorified and Exalted be He) also says:
And surely, We shall try you till We test those who strive hard (for the Cause of Allâh) and As-Sâbirûn (the patient), and We shall test your facts (i.e. the one who is a liar, and the one who is truthful).
(Part No. 7; Page No. 178) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.