Concede the legitimate worry first: a faith that never lets anything new be said can calcify into traditionalism. The New Testament still does not treat the gospel as an evolving conversation. It calls it 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.
Ask of any later movement not whether it is sincere, growing, or spiritually moving, but whether it preaches 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.”