Lukvance you are saying this:
1) Happiness is a good determinant of truth. You know that many times, people are much happier with lies ("You don't look fat in that dress." "Your puppy is in heaven chasing rabbits all day." "A man gives you a big diamond like that, he must really love you." "This lotto ticket is a sure fire winner!" )
2) If most people do something, it must be correct. You know that popular things that "everyone" does are often wrong: Slavery was a commonly accepted practice all over the world from the beginnings of human history until the 20th century. Most people in Germany went along with the Nazis. Most people in the Middle Ages went along with burning witches.
What you are implying is that the main argument for checking out Christianity is to jump on the bandwagon, not because it is actually true, but because it is popular, and seems to make people happy.
Here is what you are saying: When there were only a handful of Christians in the world, and did not make very many people happy, it must have been a lie. Nowadays, there are two billion members, and most are happy, so it must be true.
So, when, exactly did it become true?
You also ignore the fact that certain religions have more members because people who had that faith conquered and enslaved more people. The Jains and Bahai's, for example, are very peaceful religions and do not conquer or enslave anyone. They mind their own business, don't try to get their religion made into law and don't try to push conversion on others. They will never become world dominating religions that way, and might even die out. Yet, they are as happy and as likely to be true as any other religion.
If you live in Iran or India, you see very few Christians around, and those few are not happy. So, what reason would you have for investigating that religion? None, according to you. Most people around you are Muslim or Hindu and most of them are happy. Case closed.