Here's John 20:30-31, which includes the WHOLE sentence that John wrote, not just the first half.
John 20:30-31 Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.
All the other stuff that jesus did that was not recorded was not necessary by John's own admission.
The stuff he wrote down was all that is needed to come to a saving knowledge of Jesus Christ.
For that matter, the Gospel of John provides enough information about the gospel to lead someone to salvation.
Roman Catholic are masters at this.
Consider how little Scripture the average Roman Catholic reads/hears in a year and we can understand how they misunderstand the verses in question.
And if only portions of the verses are read at Mass this further dilutes their understanding of the texts.
But only if it is spoon fed to the unwise ones.
"One indeed is the universal Church of the faithful, outside which no one at all is saved, in which the priest himself is the sacrifice, Jesus Christ, whose body and blood are truly contained in the sacrament of the altar under the species of bread and wine; the bread (changed) into His body by the divine power of transubstantiation, and the wine into the blood, so that to accomplish the mystery of unity we ourselves receive from His (nature) what He Himself received from ours."
--Pope Innocent III and Lateran Council IV (A.D. 1215)
[22] When he had said this, he breathed on them; and he said to them: Receive ye the Holy Ghost. [23] Whose sins you shall forgive, they are forgiven them; and whose sins you shall retain, they are retained.