Yes you are right sis. I think the Christians who turn atheists do so because they find in difficult to understand the concept of the trinity. We, on the other hand beleive in At-Tawheed as described in Surah Ikhlaas.
Ukhti,
You bring up a very interesting and valid point. This is an area I feel many of us make a grave error when trying to bring Da'wah to Christians. All too often we use the approach of pointing out the errors in Christianity. This will not lead to them accepting Islam. They see our "proving" their belief wrong as proving to them God(swt) does not exist.
To a strong adherent of any faith, the belief in that faith is a strong glue that solidifies their belief in God(swt) if we break that bond we shatter their belief in God(swt). This is why I personally feel Da'wah needs to be showing proof that Islam is correct and should not be an approach to show them their religion is wrong.
Keep in mind a Christian strong in the belief of Christianity does have a very strong desire to serve Allaah(swt). To have them to accept Islam they need to understand Islam is the best way to serve, not have anybody prove to them their worship was wrong. Prove to them they spent a lifetime in error and you will most likely convince them Allaah(swt) does not exist.
Proving another person's belief wrong, will more likely encourage them to be an atheist, rather than guide to Islam.
Very few if any Reverts come to Islam because somebody proved to them their previous belief was wrong, they come because they learned Islam is correct. Islam is the fulfillment of the worship they been seeking, not the destruction of their years of worshiping as they saw as best.
The Evangelical door-to-door Fundies (keep in mind these Evangelical types usually believe they are the only Christians and all others who wear the name Christian are not Christian) use the approach of proving the other person is wrong. Since their main target is Christians, I suspect they are a major cause for the rise of atheism among Christians since the late 1800s. Let us not repeat the error of the Fundies by using their approach in our Da'wah efforts.
