Concede the legitimate worry first: a faith that never lets anything new be said can calcify into mere traditionalism. But that is not what is at stake here. The New Testament does not describe the gospel as an evolving conversation. It describes it as a deposit: something received, entrusted, and to be guarded intact.
Jude makes the point in a single phrase: “the faith which was once delivered unto the saints.” The word translated “once” means once-for-all, not once-upon-a-time. Paul tells Timothy to “hold fast the form of sound words” and to “keep” the “good thing which was committed unto thee.” The posture toward the apostolic gospel is custodial: you do not improve a deposit, you protect it.
This is why the question to ask of any later movement is not whether it is sincere, growing, or spiritually moving, but whether its message is the same gospel the apostles delivered: the gospel Paul says he received and passed on, “how that Christ died for our sins according to the scriptures… and that he rose again the third day.”