Hippolytus’ difference between hearing the Word and hearing the Gospel?

In Apostolic Traditions , Hippolytus (starting at section 15 through section 20 (it is too much to copy and paste)) lines out the qualifications that a catechumen must meet for acceptance as such and a 3 year period that accepted catechumens must undergo prior to being baptized. Accepted catechumens will hear the word for 3 years so long as they faithfully retain accepted catechumen behavior. > Catechumens will **hear the word for three years.** Yet if someone is > earnest and perseveres well in the matter, it is not the time that is > judged, but the conduct. Having successfully completed the catechism and been presented as baptism candidates they are allowed to hear the Gospel. > When they are chosen who are to receive baptism, let their lives be > examined, whether they have lived honorably while catechumens, whether > they honored the widows, whether they visited the sick, and whether > they have done every good work. If those who bring them forward bear > witness for them that they have done so, **then let them hear the > Gospel.** Paul seems to have presented the Gospel to Corinth as of primary importance. Hippolytus relates catechumens in 3rd Century Rome being required to "live right' and then, if it was proved that they have for three years, they could then hear the Gospel. My question is: Why would the methodology of presenting the Gospel have changed from Paul's 'presenting it as of primary importance' (see also Phillip and the Eunuch ) to that of 'prove you are worthy for 3 years' and then you can hear it? (A related question is: How could a 3 year instruction in the word be such that the Gospel is not yet heard? I understand this is probably unanswerable but I can't avoid it. Say the word and I will edit this out.)