We heard constant exhortations againt "lukewarmness" at the Kingdom Hall, and at the book study whenever we studied that part of the Revelation Climax near at hand book. Meeting attenders who rarely went out in service were painted as 'lukewarm', which is a putdown if you look at what Jesus wants to do with his lukewarm disciples in the book of Revelation (vomit them out).
When you go door to door preaching this stuff, if you took it even somewhat seriously you had to come up with reasons this stuff was true. You had to repeat it to a lot of householders. As has been pointed out by others on this forum, this is a really effective way for the Watchtower to keep its people loyal, by having them repeat the teachings over and over to non-believers. It wasn't a really effective way of making converts, but it did re-enforce those Watchtowerisms in our minds.
Going out in service was not primarily to win converts but basically to reinforce the indoctrination when you were not attending meetings. During the time I was in I totally believed it was the truth, 100%; I just thought what I didn't understand would eventually be revealed to me and that I shouldn't expect to learn everything at once and that other people in the organization who had been in the organization for years were learning new things that they had wondered about and then later it would be my turn to have things fall into place that didn't already.