The protestant will affirm the early few ecumenical council. but later, since they split so much among themselves, they have bishop/leader in their own denomination... then the followers of these leader fight with each other with different theological stands.
so it's like you can choose you can side which side with your own conscience but for the leaders, it's like 'I say this is truth and I believed it as this is what the bible says, here's my argument...' and each of them claim they're right...
since the truth isn't subjective but objective, there can't be 2 contradicting doctrines are both right.
These things may be true because we are yet carnal [immature and lack spiritual understanding]. And I must conclude myself under that.
1Co 3:1 And I, brethren, could not speak unto you as unto spiritual, but as unto carnal, even as unto babes in Christ.
1Co 3:2 I have fed you with milk, and not with meat: for hitherto ye were not able to bear it, neither yet now are ye able.
1Co 3:3 For ye are yet carnal: for whereas there is among you envying, and strife, and divisions, are ye not carnal, and walk as men?
1Co 3:4 For while one saith, I am of Paul; and another, I am of Apollos; are ye not carnal?
1Co 3:5 Who then is Paul, and who is Apollos, but ministers by whom ye believed, even as the Lord gave to every man?
1Co 3:6 I have planted, Apollos watered; but God gave the increase.
1Co 3:7 So then neither is he that planteth any thing, neither he that watereth; but God that giveth the increase.
1Co 3:8 Now he that planteth and he that watereth are one: and every man shall receive his own reward according to his own labour.
1Co 3:9 For we are labourers together with God: ye are God's husbandry, ye are God's building.
1Co 3:10 According to the grace of God which is given unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. But let every man take heed how he buildeth thereupon.
1Co 3:11 For other foundation can no man lay than that is laid, which is Jesus Christ.
To me, 1 Corinthians 3:11 is the key. The unity of the church is not in its unanimous agreement but in its foundation: Jesus Christ.
Now this Jesus Christ is clearly explained in the gospels and apostolic teachings and therefore we know who he is: the Son of God and eternal in oneness with God the Father and the Holy Spirit. Through him [the gift of grace] and by all three our salvation is secured.
The church is one because Christ is one and there is salvation by none other, nor do any other build the Church, for he said, "I will build my church."
Men may build denominations but they cannot build the church.
1Co 3:12 Now if any man build upon this foundation gold, silver, precious stones, wood, hay, stubble;
1Co 3:13 Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is.
Now these denominations, what sort are they? Are they gold, silver and precious stones? Or wood, hay, and stubble? The Bema judgment will make this known. Until then men will keep building and denominations may multiply like the empires of this world, but these will not last for eternity for they are made with the hands of men and not by God.
By its foundation the Church is actually one and cannot be divided, no matter what.