You would think that when people adopt Christianity it will bring communities closer together, strengthen families and marriages but why is it that..

Historically that's not true.
What you're observing today in predominantly Christian countries
is not the result of the religion
but the effects of capitalism promoting ego and a culture of overvaluing individuality, selfishness and materialism. It's extremely profitable.

Divorce means two households instead of one, and double the sales of appliances, furniture, and cleaning and maintenance costs. Quite often, the kids have two sets of clothes and toys, one in each home, to save toting them backwards and forwards between the parents' houses.

Workplaces prioritise employees who give their all to the job - so single adults get promoted far more than parents. When parents are absent, the kids suffer.

Christianity's gospels were not written until at least three generations after the time Jesus was thought to have existed - the year 0.
The gospels don't all agree with each other. Some stories are so markedly different that the testimony seems unreliable. Certainly it would never pass scrutiny in a modern court of enquiry.

Most religions end up developing sects with different views and interpretations of their holy books.
Most depend on hierarchies of religious leaders to interpret the text.
The leaders use their authority to accumulate power, wealth and prestige.
Whenever there are hierarchies of any kind, corruption is not far behind.
This is true of all religions and institutions, not just Christianity.

What we do have now
is far better and faster means of public communication;
it's harder to keep things secret.

the more popular a religion is, the fewer there are who have the ability to really understand what they believe in and the more people who are conformist to any popular idea of faith in society

I would say because people are not truthfully being taught about Yeshua. They are being told things that make them feel good. I don't believe they are actually taking what the whole Bible says as truth. The 10 commandments and how the father says turn away from sin and follow him. People try to twist the word to make a point that they want to make a follow accordingly.

What countries do you reference to just now becoming Christian

Good point.