I will tell you guys a story which may seem familiar:
There was once a bride and a groom. The groom had to go in voyage to prepare the wedding, and unfortunately, he could not take the bride with him. He gave his bride detailed instructions on what she had to do while he was gone. Shortly before leaving, he told her that, just after leaving, he would send his friend to remind her everything he told her. Now, the groom did not put his bride in charge, but sent his friend to remind her what to do, thus he would still be in charge; though he was not there, his instructions would still remain. Because he gave detailed instructions on what to do in any case, it’s like if he would still be there.
The bride is the Church, the groom is Jesus and His friend is the Holy Spirit. Jesus did not put His Church in charge of His Law, but gave us the Holy Spirit to remind us His Law, His Sacrifice, His teachings and His promise. To say that the Church has authority to change His Law (based on the passage of Matthew 16:19) is madness. It’s like if His Bride rebelled, and that is she’s not under the Groom’s authority. The Holy Spirit takes what is Jesus’ and gives it to us (His Word). In this age, the Holy Spirit cannot change the Law, because he does not speak from himself, but from God/Jesus. The Holy Spirit is the Spirit of Truth. he only says what the Groom said. If he would speak from himself, he would be a liar. Therefore, Jesus did not put His Church in charge, or in His place. He sent the Holy Ghost (the Holy Spirit and Jesus are the same, based on the Trinity) to remind us that there is still a Law which we must follow. If the Holy Spirit is here, it means that Jesus is spiritually here as well and saying that you have the authority to change His Word, is like rebelling against Him under His surveillance.
Saying that you can change His Word is like making yourself equal with Him, or superior, which is blasphemy.