Thank you, Lord Jesus. Things are now much clearer with the help of several online friends who answered my query on "open interpretation" and the "invisible universal church". One of them, E., was especially cogent in his argument for 'a visible universal church'. Below are mostly what he had written:
'IF YOU WANTED TO CONTINUE to continue the dialogue with your Protestant colleague, then you would have to offer scriptural arguments against the idea of an invisible universal church.
'You were right to quote Matthew 16:18-19. Insist that Jesus founded His Church on Peter, a visible person, who was given the authority to govern. Next, you can point your friend to Matthew 18:15-18, where Jesus tells the Apostles how to deal with sinners:
' "If your brother sins (against you), go and tell him his fault between you and him alone. If he listens to you, you have won over your brother. If he does not listen, take one or two others along with you, so that 'every fact may be established on the testimony of two or three witnesses.' If he refuses to listen to them, tell the church. If he refuses to listen even to the church, then treat him as you would a Gentile or a tax collector."
'Jesus taught that sinners should be brought to the church as a last resort. Now, this clearly presupposes that the church has to be a visible and living authority, not some vague invisible entity.'
'LASTLY, S. ALSO MADE A GOOD POINT. You can guide your colleague through Acts 15, where the church in Antioch decided to consult the apostles & the elders -- visible church authority, when they had some confusion about doctrines, specifically about the need for circumcision.
' "Some who had come down from Judea were instructing the brothers, 'Unless you are circumcised according to the Mosaic practice, you cannot be saved.' Because there arose no little dissension and debate by Paul and Barnabas with them, it was decided that Paul, Barnabas, and some of the others should go up to Jerusalem to the apostles and presbyters about this question." (Acts 15:1-2)
'Again, the church can be seen to be a visible and universal authority, as the apostles & elders in Jerusalem exercised their teaching authority over the believers in Antioch, Syria & Cilicia.
'Then the apostles and presbyters, in agreement with the whole church, decided to choose representatives and to send them to Antioch with Paul and Barnabas. The ones chosen were Judas, who was called Barsabbas, and Silas, leaders among the brothers. This is the letter
delivered by them: "The apostles and the presbyters, your brothers, to the brothers in Antioch, Syria, and Cilicia of Gentile origin: greetings. Since we have heard that some of our number (who went out) without any mandate from us have upset you with their teachings and disturbed your peace of mind,... 'It is the decision of the holy Spirit and of us not to place on you any burden beyond these necessities, namely, to abstain from meat sacrificed to idols, from blood, from meats of strangled animals, and from unlawful marriage. If you keep free of these, you will be doing what is right. Farewell.' " (Acts 15: 22-24, 28-29)'
(See also Scripture and Tradition and Try Reasoning With A....)
And a good point from another E.: "Christ and the disciples had worked so hard to stamp out heresy and false teaching, didn't he think that Christ would want to set up a hierarchy or authority to prevent the followers from being led astray after that?"
So it is! Why did i forget to mention this? Finally, didn't St. Peter write about this twice in his second letter:
"First of all you must understand this, that no prophecy of scripture is a matter of one's own interpretation, because no prophecy ever came by human will, but men and women moved by the Holy Spirit spoke from God." -- 2 Peter 1:20-21
"... So also our beloved brother Paul wrote to you ... in all his letters. There are some things in them hard to understand, which the ignorant and unstable twist to their own destruction, as they do the other scriptures." -- 2 Peter 3:15-16
The challenge now is "how to bear witness with Christian charity". As St. Peter had written in his first letter: "Always be ready to give an explanation to anyone who asks you for a reason for your hope, but do it with gentleness and reverence, keeping your conscience clear, so that, when you are maligned, those who defame your good conduct in Christ may themselves be put to shame." (1 Peter 3:15b-16)