Good Morning Ireneus,
What guarantee is there for those things to be true especially in view of the fact Jesus himself foretold truth will be overpowered by interpolators and false teachers who will be operating in “his name”
We know they are true because your can read it for yourself in the Word of God. If anyone teaches a doctrine that is new...alarm bells go off.
If anyone teaches a doctrine of new light....alarm bells go off.
If anyone says you have to defend God's name Jehovah to get saved instead of coming to Christ .... alarm bells go off.
If anyone says you have to belong to their organization in order to get right with God ... alarm bells go off.
Jesus said, "Come to Me, and I will give you rest". We get into trouble when we seek to do business with anyone other than Jesus, however, good reasonable, logical, righteous, or otherwise appealing it may seem. It is WRONG, WRONG, WRONG.
My "sheep hear my voice" .... "another voice they simply will not follow".
"I am the door" ... if anyone seeks to get in by some other way, he is a thief and a robber" - Jesus
"though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. Gal. 1: 8
Everything has been frozen with the passing of the apostles. You can read it for yourself in the bible. Every now and then you can do a search for just about any doctrine and add the phrase "early church fathers" to your google search.
That phrase sounds catholic I know, but it is the phrase the gets you to the early congregation elders and what they understood the apostles to mean. They wrote a LOT of stuff because the heretics of the day were putting forth all kinds of stuff designed to lead people away from the "sufficiency of Christ" Several were students of the apostles themselves.
But the bible is the primary source, by far.