Then why deny that that same Holy Spirit was given to EVERY believer?
Acts 15:8 And God, which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as he did unto us; 9 And put no difference between us and them, purifying their hearts by faith.
It's not just some hierarchical "magisterium" that has the ability to understand what scripture teaches.
1 John 2:27 But the anointing which ye have received of him abideth in you, and ye need not that any man teach you: but as the same anointing teacheth you of all things, and is truth, and is no lie, and even as it hath taught you, ye shall abide in him.
As a matter of fact we should check with scripture to see if what they teach is truth.
Acts 17:11 These were more noble than those in Thessalonica, in that they received the word with all readiness of mind, and searched the scriptures daily, whether those things were so.
If Paul commended them for checking even his teaching how much more so some "magisterium"? Yet you take the word of some "ecumenical council" without checking with scripture?
It's not a matter of denying anything. Notice to whom He is speaking when He speaks. All through His ministry, through His Passion and Resurrection. The Day of Pentecost.
It's always to some or all of the Apostles. Paul's epistles are addressed to churches. The common thread is that He deals with groups, not individuals. The pattern is OT and confirmed by Paul to the church at Corinth..."at the mouth of two or three witnesses..."
The first council in Acts 15 has been discussed here many times. There was a pattern laid down. So there is certainly scripture to support the convening of the bishops to seek the Holy Spirit about disputed matters.
Transferring everything committed and promised to the Church to the individual is going to create confusion. Beyond puzzling over why Jesus did not tell the Apostles to write it all down, I ask what about the Christians who had no New Testament? Answer: They were to be in the Church and as Paul says over and over, act and believe like HE had done told them to act. and believe
When, exactly did the common practice of the early church, as shown above, get negated? By what authority?