How did we lose them? From your post it sounds like you mean converts to evangelical Christianity.
The answer is, IMO, rather surprising when you look deeply at it. We lose them the same way that the evangelical communities lose them in turn a decade or two down the road. Bear with me.
It doesn’t sound charitable, but the facts on the ground suggest that the sort of “evangelical experience Christianity” you are referring to is the exit ramp from Christianity that leads to secularism.
Here’s the process I’ve observed (first hand as I belonged to such a protestant group in college):
- Nominal Christian with a lukewarm faith at most and little to no real Christian community support in his own life experiences enthusiastic evangelical believer and is intrigued.
- He then is introduced by the evangelist to the evangelist’s community and is really intrigued. After a time immersed in this ‘fellowship’ he conflates enthusiasm and faith and concludes that he never himself previously had any faith. Maybe it’s even true.
- He decides that he never had faith because he was brought up in a polluted version of Christianity and decides to leave and go where the “real believers” are so that he can be “fed by the Word.” What he really means is keep experiencing God in a community of believers. He’s simply persuaded that they got that way via their diligent attention to Scripture.
- Over time, the enthusiasm fades and he tries desperately to prop it up because he’s built his faith on feeling that enthusiasm. They do rock beat worship music, do evangelism rallies with light shows, they parade a non-stop litany of conversion testimonies to stoke the flames.
- Eventually, the enthusiasm fades anyways and the props no longer keep it up. Maybe he jumps to a new church. More often he grows in frustration and bitterness takes root. God no longer seems to answer his prayers. He drops out of activities. Eventually, he stops going to church. He becomes cynical about the “religious right” and his conscience brings to light many of the serious over-simplifications and intellectual dishonesty that exists in the American evangelical world (7 day creationism, biblical literalism in general, actual homophobia, absurd prohibitions on playing cards, alcohol and dancing).
- He tosses baby Jesus out with the bathwater of American evangelicalism and becomes just another secularist.
This isn’t just me pontificating. The churn rate at many of the non-denom ‘churches’ is extremely high, look it up. It’s because they build the person’s entire perception of what faith is on the emotional surge of initial conversion. When the feeling fades (as it always does, even for saints like Mother Theresa), so does the faith.
So as I said, the thing he left for is also the thing he’ll leave the new place because of, eventually. He just doesn’t realize that he is leaving the Lamb and veggies of Catholicism for the Big Mac & fries of Flashinthepan Community Church. Even the congregations themselves follow that pattern. Look at Schuller’s Crystal Cathedral. He pulled 'em in, got 'em riled up and enthused and had nothing more to offer when the enthusiasm dulled. No sacraments, no millennia old churches.