Quite so. And that's one of the many reasons why we can't rely on written Scriptures alone. You yourself said it well with the quote you selected:
"For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God." -- 1 Corinthians 1:18
Many are the saved who have never read a word of Scripture, never read a word of anything, let alone interpreted it. They learned about God through other people who are in Christ--- through preaching --- through the Church.
Do you think it was pointless, useless, unnecessary, for Christ to build this Church, and in it to appoint some --- but not all --- as teachers?
What is being "preached?" The Scriptures, the inerrant word of God.
That the man of God may be perfect, throughly furnished unto all good works. " -- 2 Timothy 3:16-17"All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
Sounds pretty all-inclusive to me. What more is there to know of our salvation that is not found in Scripture?
God chose to speak to men through the gift of His Holy Spirit who illuminates His holy word.
The church is a group of believers who have been so blessed who glorify God in their understanding and their worship.
Do you think it was pointless, useless, unnecessary, for Christ to build this Church
Christ came to redeem His flock. Believers are "lively stones," not mortar and brick.
to appoint some --- but not all --- as teachers?
Of course God appointed teachers and pastors and elders and all believers.
God did not, however, appoint "another Christ" or a "co-redeemer" or a "dispenstrix of grace."
Neither the church nor the magisterium nor Mary dispenses God's grace. The church is made up of those who have received God's grace through faith in His Son, according to God's good pleasure.