Its an outright error, actually, religious tolerance (which is religious indifrentism in practice) was formally condemned in the Syllabus of Errors.
In fact, religious pluralism is by definition retarded, since 99% of religions (especially Atheism) say the other 99% of religions are false and their precepts are sinful. Therefore it is a logical inconsistency to even believe religious co-existence and "ecumenism" are possible unless no one actually practices their faith. The only reason people believe religious indifferentism is because it *feels* good to pretend that we can all get along. Its all feeling based, theres absolutely zero rationality in it.
Be careful with your language here. The Syllabus of Errors condemns religious indifferentism and religious liberty. Religious tolerance, on the other hand, which means tolerating those who practice false religions, is actually consistent with Church teaching. For example, Sicut Judaeis Non says we must tolerate the Jews practicing their false religion, and we are not allowed to forcibly convert them into Catholicism. Similarly, we cannot forcibly convert anyone of any false religion. It may seem like I am nitpicking here, and apologies for that, but this is my first article in my 3 article series on religious indifferentism, religious liberty, and religious tolerance, and they are 3 distinct things.
Unfortunately, most people will be damned. Christ Himself says the path to Hell is wide and most go down it and the path to Heaven is narrow and few find it. He also says not all you profess His name will go to Heaven. He also says that He is the only way to the Father in Heaven.
I do like Martin Luther. He is correct about a few things.
According to this Protestant theology, a true Christian has been possessed by the Holy Spirit. Your salvation has thereby been guaranteed, as you have become a fleshly vessel of God.
Those who genuinely call on the Lord God are rewarded this. The Holy Spirit descends and you are freed from death. Of course, we can all name a hundred false Christians. They were never genuine to begin with; as they were blatant sinners and thereby could not have been possessed by the Spirit of God.
However, all sin is equal. There are no levels to sin. Eating too much is akin to child sacrifice. Drinking a little too much wine is akin to murder.
So according to Protestant Theology, if you cast your wandering gaze onto a fair maid... Sorry! NOT a *real* Christian and you never were! Your moment of simple lust has uncovered you to be a monster.
----------------
So the question is this... Is Luther right? Are none of us Christians? Is a true Christian absolutely without sinfulness or desires of the flesh?
What I just described is Protestant theology without the endless rhetoric, gaslighting and misdirection. Don't believe me? Go tell a "Christian" that all sin is not equal. Tell them that salvation is not guaranteed just because you confessed Christ and got baptized. Tell them that one can fall from God's favor because of sin and be rejected.
Martin Luther was right about Sola Fide. *Work* is justified by Faith alone. He was right about the jews.
Otherwise, Catholicism is the closest doctrine to the truth.