No man nor visible church can "keep the faith entire." All men sin, even priests and popes and every member of every congregation. Nobody gets it all right.
So we search for the church which most closely adheres to the written word of God handed down for centuries and preserved by God for His glory.
And that's where Rome fails because Rome teaches and practices so much that is antithetical to the truth found in Scripture. It's just that simple.
Rome errs in believing priests are "another Christ." Rome errs when it encourages men to pray to dead people who may or may not be now in heaven. Rome errs when it confers near-divinity to Mary by calling her a "mediatrix" and "co-redeemer." Rome errs when it says good works are required to earn salvation. Rome errs when it says its bishop of Rome is head of the church and speaks infallibly on matters of faith. Rome errs when it says its magisterium is likewise infallible and is the only arbiter of the Scriptures which are subject to that group of men. Rome errs when it believes baptism regenerates. Rome errs when it says the Lord's Supper is a re-sacrifice of Christ. And Rome errs when it confuses justification with sanctification, thereby telling men their own piety is their ticket to paradise when the ONLY thing that saves anyone is Christ's work on the cross freely and mercifully imputed to the believer who was named by God from before the foundation of the world.
Thus the necessity of the Reformation -- to right the listing church of Jesus Christ on earth.
I'm Presbyterian. I believe church structure and accountability and governance are beneficial to the earthly institution of God's people. But every speck of that structure and governing is subject to the word of God. Or else it is false.
No doubt there are true Christians in the Roman Catholic church who love Christ beyond measure, but that will not be because of Rome's teaching, but in spite of it.
Clear, concise, and true.
Bless you.