As the tittle of the blog suggest, this question has been among the biggest question of all, especially for those who are religious.In the following paragraphs I will try to analyze it best to my understanding.
To begin with, we see many common things in all religions.Each religion has a founder,a historical origin, a holy scripture,a set of rituals and beliefs, ceremonial functions, a set of ideas regarding origin of life,death,salvation.Furthermore, since all of us are seeking same state of living which is a state of peace and contentment, it would mean we have the same problem.Hence, the solution would be the same for all of us.
The problem arises when we see all the different religions being followed. Now the question comes, if our desire is the same which is to attain salvation and if the desire is same, the underlying problem must also be the same causing us to desire salvation.But how come salvation can be attained by different methods when the desire is the same and so is the problem.
Thus, throughout the ages we have seen conflicts among different religions each claiming to be the right way to GOD.In fact, religions like Christianity and Islam often claims to have exclusive right over Truth and even consider prioritization or religious conversions to be an essential part of their religion.
But the question remains, how can one claim if one religion is right and the other is wrong.
Usually, we claim and refer to our scriptures as the authentic source and basis of our religion but each religion has a scripture.Since all our beliefs, rituals,practices, ceremonies comes straight out of the scriptures therefore, the scriptures of all religions are at scrutiny or in question by those who consider their religion to be the only way and so their entire efforts goes to prove insufficiency or inadequacy in other scriptures.Once they do that, that itself makes all other beliefs, practices,rituals of that particular religion invalid as all those depends on the scripture itself.
But in the middle of this chaos as we all wonder and try to figure out and spread our message to rest of the world,GOD is lost.
Now coming back to things which are common to all of us irrespective of our religion, we all agree GOD is One,All Knowing,Eternal,Self Existent,Beyond our logic and reasoning.Beyond our physical senses.If we agree on these features of GOD then in an instant all our claims, all our reasons, logic fails.
Please Read With More Attention from here.
With our ability to reason and logic we developed this world,from bullock carts we made into Motor Cars, from it we moved on to flying and even in space.With our logic and reasoning we developed cities, buildings, ways to do business, services, governments.All the things we have around us today. All the things with the ability of our logic and reasoning.Furthermore,we have discovered about our ancient civilizations and looked into our future advances through logic and reasoning.Now all these developments were possible because we could see the physical world, our senses could feel them and our logic could comprehend them. Now as mentioned above since GOD goes beyond our senses and reason/logic that put a limit on ourselves to completely understand GOD as well as God's word (Holy Scriptures). Thus, anyone who claims that his/her religion is the only way to salvation put himself/herself in dilemma.Since as a human we are limited so are our logic and reason.Therefore, GOD cannot be understood with our reason or logic but if someone claims so then he/she makes GOD no different than rest of the physical world which is not only understood by us but we have manipulated it for our comfort. Since, during most interfaith debates people try to make sense, try to reason and logic about the authenticity of each other's scriptures the whole process falls into fallacy since neither GOD nor God's word are within the logic and reasoning of humans and this limitation of us itself makes us mortal.Therefore, no claim by anyone would be enough as there is no way to know the depth to which God or God's word can go.
The answer to the question why there are so many religions if there is only one GOD itself is rooted in the above mentioned paragraphs. Since GOD is limitless,infinite,beyond logic,beyond reason,beyond physical senses so are God's ways and that itself prove God's Majesty as there can be no set of ideas,thoughts,religions,holy scriptures,rituals,beliefs,practices,ceremonies which can ever confine GOD.In fact, to say that the Infinite GOD has a finite way to reach itself is a contradictory.
Thank You.
Hope the following lines of Siri Guru Granth Sahib Ji answer this age old question.
1.
ਸਰਬ ਧਰਮ ਮਿਹ ਸਰ੍ੇਸਟ ਧਰਮੁ ॥
sarab Dharam meh saraysat Dharam.
Of all religions, the best religion
ਹਿਰ ਕੋ ਨਾਮੁ ਜਿਪ ਿਨਰਮਲ ਕਰਮੁ ॥
har ko naam jap nirmal karam.
is to chant the Name of the Lord and maintain pure conduct.
ਸਗਲ ਿਕਰ੍ਆ ਮਿਹ ਊਤਮ ਿਕਿਰਆ ॥
sagal kir-aa meh ootam kiri-aa.
Of all religious rituals, the most sublime ritual
ਸਾਧਸੰਿਗ ਦੁਰਮਿਤ ਮਲੁ ਿਹਿਰਆ ॥
saaDhsang durmat mal hiri-aa.
is to erase the filth of the dirty mind in the Company of the Holy.
ਸਗਲ ਉਦਮ ਮਿਹ ਉਦਮੁ ਭਲਾ ॥
sagal udam meh udam bhalaa.
Of all efforts, the best effort
ਹਿਰ ਕਾ ਨਾਮੁ ਜਪਹ ੁ ਜੀਅ ਸਦਾ ॥
har kaa naam japahu jee-a sadaa.
is to chant the Name of the Lord in the heart, forever.
ਸਗਲ ਬਾਨੀ ਮਿਹ ਅੰਿਮਰ੍ਤ ਬਾਨੀ ॥
sagal baanee meh amrit baanee.
Of all speech, the most ambrosial speech
ਹਿਰ ਕੋ ਜਸੁ ਸੁਿਨ ਰਸਨ ਬਖਾਨੀ ॥
har ko jas sun rasan bakhaanee.
is to hear the Lord's Praise and chant it with the tongue.
ਸਗਲ ਥਾਨ ਤੇ ਓਹ ੁ ਊਤਮ ਥਾਨੁ ॥
sagal thaan tay oh ootam thaan.
Of all places, the most sublime place,
ਨਾਨਕ ਿਜਹ ਘਿਟ ਵਸੈ ਹਿਰ ਨਾਮੁ ॥੮॥੩॥
naanak jih ghat vasai har naam. ||8||3||
O Nanak, is that heart in which the Name of the Lord abides. ||8||3||
2.ਿਵਸੁਏ ਚਿਸਆ ਘੜੀਆ ਪਹਰਾ ਿਥਤੀ ਵਾਰੀ ਮਾਹ ੁ ਹੋਆ ॥
visu-ay chasi-aa gharhee-aa pahraa thitee vaaree maahu ho-aa.
The seconds, minutes and hours, days, weeks and months,
ਸੂਰਜੁ ਏਕੋ ਰਿੁ ਤ ਅਨੇਕ ॥
sooraj ayko rut anayk.
and the various seasons originate from the one sun;
ਨਾਨਕ ਕਰਤੇ ਕੇ ਕੇਤੇ ਵੇਸ ॥੨॥੨॥
naanak kartay kay kaytay vays. ||2||2||
O Nanak, in just the same way, the many forms originate from the Creator. ||2||2||
To begin with, we see many common things in all religions.Each religion has a founder,a historical origin, a holy scripture,a set of rituals and beliefs, ceremonial functions, a set of ideas regarding origin of life,death,salvation.Furthermore, since all of us are seeking same state of living which is a state of peace and contentment, it would mean we have the same problem.Hence, the solution would be the same for all of us.
The problem arises when we see all the different religions being followed. Now the question comes, if our desire is the same which is to attain salvation and if the desire is same, the underlying problem must also be the same causing us to desire salvation.But how come salvation can be attained by different methods when the desire is the same and so is the problem.
Thus, throughout the ages we have seen conflicts among different religions each claiming to be the right way to GOD.In fact, religions like Christianity and Islam often claims to have exclusive right over Truth and even consider prioritization or religious conversions to be an essential part of their religion.
But the question remains, how can one claim if one religion is right and the other is wrong.
Usually, we claim and refer to our scriptures as the authentic source and basis of our religion but each religion has a scripture.Since all our beliefs, rituals,practices, ceremonies comes straight out of the scriptures therefore, the scriptures of all religions are at scrutiny or in question by those who consider their religion to be the only way and so their entire efforts goes to prove insufficiency or inadequacy in other scriptures.Once they do that, that itself makes all other beliefs, practices,rituals of that particular religion invalid as all those depends on the scripture itself.
But in the middle of this chaos as we all wonder and try to figure out and spread our message to rest of the world,GOD is lost.
Now coming back to things which are common to all of us irrespective of our religion, we all agree GOD is One,All Knowing,Eternal,Self Existent,Beyond our logic and reasoning.Beyond our physical senses.If we agree on these features of GOD then in an instant all our claims, all our reasons, logic fails.
Please Read With More Attention from here.
With our ability to reason and logic we developed this world,from bullock carts we made into Motor Cars, from it we moved on to flying and even in space.With our logic and reasoning we developed cities, buildings, ways to do business, services, governments.All the things we have around us today. All the things with the ability of our logic and reasoning.Furthermore,we have discovered about our ancient civilizations and looked into our future advances through logic and reasoning.Now all these developments were possible because we could see the physical world, our senses could feel them and our logic could comprehend them. Now as mentioned above since GOD goes beyond our senses and reason/logic that put a limit on ourselves to completely understand GOD as well as God's word (Holy Scriptures). Thus, anyone who claims that his/her religion is the only way to salvation put himself/herself in dilemma.Since as a human we are limited so are our logic and reason.Therefore, GOD cannot be understood with our reason or logic but if someone claims so then he/she makes GOD no different than rest of the physical world which is not only understood by us but we have manipulated it for our comfort. Since, during most interfaith debates people try to make sense, try to reason and logic about the authenticity of each other's scriptures the whole process falls into fallacy since neither GOD nor God's word are within the logic and reasoning of humans and this limitation of us itself makes us mortal.Therefore, no claim by anyone would be enough as there is no way to know the depth to which God or God's word can go.
The answer to the question why there are so many religions if there is only one GOD itself is rooted in the above mentioned paragraphs. Since GOD is limitless,infinite,beyond logic,beyond reason,beyond physical senses so are God's ways and that itself prove God's Majesty as there can be no set of ideas,thoughts,religions,holy scriptures,rituals,beliefs,practices,ceremonies which can ever confine GOD.In fact, to say that the Infinite GOD has a finite way to reach itself is a contradictory.
Thank You.

Hope the following lines of Siri Guru Granth Sahib Ji answer this age old question.
1.
ਸਰਬ ਧਰਮ ਮਿਹ ਸਰ੍ੇਸਟ ਧਰਮੁ ॥
sarab Dharam meh saraysat Dharam.
Of all religions, the best religion
ਹਿਰ ਕੋ ਨਾਮੁ ਜਿਪ ਿਨਰਮਲ ਕਰਮੁ ॥
har ko naam jap nirmal karam.
is to chant the Name of the Lord and maintain pure conduct.
ਸਗਲ ਿਕਰ੍ਆ ਮਿਹ ਊਤਮ ਿਕਿਰਆ ॥
sagal kir-aa meh ootam kiri-aa.
Of all religious rituals, the most sublime ritual
ਸਾਧਸੰਿਗ ਦੁਰਮਿਤ ਮਲੁ ਿਹਿਰਆ ॥
saaDhsang durmat mal hiri-aa.
is to erase the filth of the dirty mind in the Company of the Holy.
ਸਗਲ ਉਦਮ ਮਿਹ ਉਦਮੁ ਭਲਾ ॥
sagal udam meh udam bhalaa.
Of all efforts, the best effort
ਹਿਰ ਕਾ ਨਾਮੁ ਜਪਹ ੁ ਜੀਅ ਸਦਾ ॥
har kaa naam japahu jee-a sadaa.
is to chant the Name of the Lord in the heart, forever.
ਸਗਲ ਬਾਨੀ ਮਿਹ ਅੰਿਮਰ੍ਤ ਬਾਨੀ ॥
sagal baanee meh amrit baanee.
Of all speech, the most ambrosial speech
ਹਿਰ ਕੋ ਜਸੁ ਸੁਿਨ ਰਸਨ ਬਖਾਨੀ ॥
har ko jas sun rasan bakhaanee.
is to hear the Lord's Praise and chant it with the tongue.
ਸਗਲ ਥਾਨ ਤੇ ਓਹ ੁ ਊਤਮ ਥਾਨੁ ॥
sagal thaan tay oh ootam thaan.
Of all places, the most sublime place,
ਨਾਨਕ ਿਜਹ ਘਿਟ ਵਸੈ ਹਿਰ ਨਾਮੁ ॥੮॥੩॥
naanak jih ghat vasai har naam. ||8||3||
O Nanak, is that heart in which the Name of the Lord abides. ||8||3||
2.ਿਵਸੁਏ ਚਿਸਆ ਘੜੀਆ ਪਹਰਾ ਿਥਤੀ ਵਾਰੀ ਮਾਹ ੁ ਹੋਆ ॥
visu-ay chasi-aa gharhee-aa pahraa thitee vaaree maahu ho-aa.
The seconds, minutes and hours, days, weeks and months,
ਸੂਰਜੁ ਏਕੋ ਰਿੁ ਤ ਅਨੇਕ ॥
sooraj ayko rut anayk.
and the various seasons originate from the one sun;
ਨਾਨਕ ਕਰਤੇ ਕੇ ਕੇਤੇ ਵੇਸ ॥੨॥੨॥
naanak kartay kay kaytay vays. ||2||2||
O Nanak, in just the same way, the many forms originate from the Creator. ||2||2||