A non Muslim friend of mine asked how I could prove the existence of God and why He has given us life, and what its purpose is. My answer did not satisfy him, please tell me what I should tell him?

Praise be to Allah

My dear Muslim brother, your efforts to call people to Allah
and explain the reality of Allah’s existence make us very happy. Finding out
about Allah is in tune with the sound fitrah (natural inclinations of man)
and with sound reasoning. How many there are who, once the truth becomes
clear to them, they hasten to submit to Allah (enter Islam). If each one of
us was to do his duty towards his religion, a great deal of good would be
achieved. So we congratulate you, our brother, for undertaking the mission
of the Prophets and Messengers, and we give you glad tidings of the great
reward which you are promised, as your Prophet (peace and blessings of Allah
be upon him) said: “If Allah were to guide one man at your hands, that would
be better for you than red camels.” (al-Bukhaari,Muslim). “Red camels” are
the best kind of camels. 

Secondly: 

With regard to proof of the existence of Allah, it is obvious
to anyone who ponders the matter, and there is no need for a lengthy
discussion. When we ponder the matter, we find that it is divided into three
categories: instinctive evidence, tangible evidence and shar’i (revelatory)
evidence. We will explain this to you further, insha Allah. 

1 – Instinctive evidence: 

Al-Shaykh Ibn ‘Uthaymeen said: 

The instinctive evidence that God exists is the strongest of
all evidence for those who are not led astray by the devils. Hence Allah
says (interpretation of the meaning): 

“So set you (O Muhammad) your face towards the religion (of
pure Islamic Monotheism) Haneef (worship none but Allah Alone). Allah’s
Fitrah (i.e. Allah’s Islamic Monotheism) with which He has created mankind.”
[30:30] 

Man’s sound nature (fitrah) testifies to the existence of God
and man cannot turn away from this unless the devils mislead him; whoever is
misled by the devils may not recognize this evidence.” (From Sharh
al-Safareeniyyah) 

Every person feels inside himself that he has a Lord and
Creator, and he feels that he is in need of Him; if some major calamity
befalls him he turns his hands, eyes and heart towards the heavens, seeking
help from his Lord. 

2 – Tangible evidence: 

This refers to the things that exist in this universe; we see
around us things that exist, such as trees, rocks, mankind, the earth, the
heavens, seas, rivers… 

If it is asked: these things are so many – who created them
and is taking care of them? 

The answer is that if these things came into being by
accident, spontaneously and with no cause, then there is no one who knows
how they were created, and that is one possibility. But there is another
possibility, which is that these things created themselves and are taking
care of themselves. And there is a third possibility, which is that there is
Someone Who created them. When we look at these three possibilities, we find
that the first and the second are impossible. If we reject the first and the
second, then the third must be the one which is correct, which is that these
things have a Creator Who created them, and that Creator is Allah. This is
what is stated in the Quran, where Allah says (what means): 

“Were they created by nothing? Or were they themselves the
creators?

Or did they create the heavens and the earth? Nay, but they
have no firm Belief.” [52:35] 

Moreover, when were these mighty things created? For all
these years, who is it that has decreed that they should remain in this
world and has granted them the means of abiding? 

The answer is it is Allah who has given to each thing that
which is suited to it and will guarantee its survival. Do you not see the
beautiful green plants; when Allah cuts off their water supply, can they
live? No, rather they become dry stalks. If you ponder all things you will
find that they are dependent upon Allah. Were it not for Allah, nothing
would remain. 

And Allah has created everything to do that for which it is
suited. So camels, for example, are for riding. Allah says (interpretation
of the meaning): 

“Do they not see that We have created for them of what Our
Hands have created, the cattle, so that they are their owners.

And We have subdued them unto them so that some of them they
have for riding and some they eat.” [36:71-72]

Look at the camel and how Allah has created it strong, with a
strong back, so that it can be used for riding and it is able to endure
harsh conditions which other animals cannot bear.

If you look at other creatures you will find that they are
suited to the purposes for which they were created. Glory be to Allah.

Other examples of tangible evidence include the following:

When calamities befall people this points to the existence of
the Creator, for example, when they call upon Allah and Allah responds to
their prayer; this points to the existence of Allah. Al-Shaykh Ibn
‘Uthaymeen said: “When the Prophet (peace and blessings of Allah be upon
him) prayed for rain, he said, ‘Allahumma aghithna, Allahumma aghithna (O
Allah, send us rain, O Allah, send us rain).’ Then a cloud came and it
started to rain before he had even come down from the minbar (pulpit). This
points to the existence of the Creator.” (Sharh al-Safareeniyyah).

 3 – Shar’i evidence:

 All divinely-revealed laws point to the existence of Allah.
Al-Shaykh Ibn ‘Uthaymeen said:

“All the divinely-revealed laws point to the existence of the
Creator and to the perfect nature of His knowledge, wisdom and mercy,
because these laws must have been prescribed by someone, and that Lawgiver
is Allah.” (From Sharh al-Safareeniyyah).

With regard to your question: Why did Allah create us?

The answer is: so that we would worship Him, thank Him and
remember Him, and do that which He has commanded us. You know that among
mankind there are disbelievers and there are Muslims. This is because Allah
wants to test His slaves as to whether they will worship Him or worship
others. That is after Allah has showed the way to everyone. Allah says
(interpretation of the meaning):

“Who has created death and life that He may test you which of
you is best in deed.” [67:2]

“And I (Allah) created not the jinn and mankind except that
they should worship Me (Alone).” [51:56]

We ask Allah to enable us and you to do that which He loves
and is pleased with, and to do more da’wah (propagation) and work for the
sake of His religion.

May Allah send blessings and peace upon our Prophet Muhammad.

And Allah knows best.