Here are their most serious errors:

1. Catholic Error: Mary-worship

Catholics often misquote the verse below to claim “Mary-worship”.

(Luke 1:28)

New American Standard Bible
And coming in, he said to her, “Greetings, favored one! The Lord is with you.”

King James Bible
And the angel came in unto her, and said, Hail, thou that art highly favoured, the Lord is with thee: blessed art thou among women.

As we see, “Hail” = “Greetings”.
The angel is talking to Mary not worshipping her.
Mary is just a human being and she can’t be worshipped.

The rule to determine doctrine & refute the traditions of men is simple:

“If it’s not found in Holy Scripture, don’t do it; for if you do, you’re adding to His Word”.

In other words, “only do things commanded or written in the Bible”.

2. Catholic error: Angel worship

There are some Catholics who are fond of angel worship when such a thing is non-existent in Holy Scripture.

Ask yourself a simple question (instead of getting angry with me for speaking the truth), “show one verse in the Bible which depicts angel worship”? There is “none”.

So we cannot do religious acts which are not found/or are against the Bible and then call ourselves as Bible-Obeying-Christians.

In fact, the very “opposite” verse exists “literally” which implies “don’t worship an angel” but yet the traditions of men teach contrary to it. Here’s that verse:

“At this I (apostle John) fell at his feet to worship him (the angel). But he (the angel) said to me, “Don’t do that! I am a fellow servant with you and with your brothers and sisters who hold to the testimony of Jesus. Worship God! For it is the Spirit of prophecy who bears testimony to Jesus.” (Revelation 19:10)

P/S:
No verse, no talk to determine the doctrines of Truth. Remember, it’s written that “don’t do that”.

His Words will condemn your false teachings (which have no verses ‘literally’) especially in Christianity.

Remember, the “Bible verse” below states that “angel worship” is “not a sign of humility” but rather a sign of “false humility” and “may disqualify you”:

“Do not let anyone who delights in false humility and the worship of angels disqualify you. Such a person also goes into great detail about what they have seen; they are puffed up with idle notions by their unspiritual mind.” (Colossians 2:18)

Note also that the verse above “literally states” that those who do “angel worship” often have “seen” something (e.g. A vision of something like that but the Bible already states in the verse above that these are false).

3. Catholic error: Saint worship

Saints are not to be worshipped either since they are creation and not the Creator (saint-worship ‘verses’ with examples are ‘not’ found in the Bible).

The previous verse in Revelation 19:10 clearly states that “Worship God” meaning “only God can be Worshipped”. Here it is again:

Lord Jesus Christ is “God Who came in Flesh” and hence received Worship whilst on earth itself (note that neither Lord Jesus nor the Bible ‘corrects’ those who worshipped Him).

Before Resurrection (Christ received worship with no corrections – implying He is God in Flesh):

“And he said, “Lord, I believe.” And he worshiped Him.” (John 9:38)

After Resurrection (Christ received worship with no corrections – implying He is God in Flesh):

“When they saw Him (Lord Jesus Christ), they worshiped Him; but some were doubtful.” (Matthew 28:17)

P/S:

There is “no verse” which states men or saints can be worshipped but only God (Revelation 19:10). And since Christ received worship (with no corrections in the Bible that such an act is wrong, both before/after His Resurrection, John 9:38 & Matthew 28:17), Lord Jesus Christ is God in Flesh (Who became Man; not the other way around).

4. Catholic error: Prayer “to” the saints including Mary.

#Fact: Prayer is to be directed to God alone (for there is ‘no’ mediator between men and God, except Christ Jesus our Lord as the verse below states ‘literally again’ – meaning ‘no’ Mary or any other saint can stand on behalf or be a mediator of any sort):

“For there is one God and one mediator between God and mankind, the man Christ Jesus,” (1 Timothy 2:5 – sorry, but there’s ‘no’ mention or possibility of any sub-mediators be it Mary, Angels or the Saints — remember, “no adding” to His Word).

Also, the verse below is often misquoted by certain Catholics to seemingly prove the act of praying ‘to’ the saints. In fact, as the verse itself shows, it’s actually the prayers ‘of’ the saints (originating from them) and not the prayers ‘to’ the saints (directed to them) which is talked about. Note that all these prayers & incense are ‘directed to God’ (not any other creation):

“And the smoke of the incense, with the prayers of the saints, went up before God out of the angel’s hand.” (Revelation 8:4)

Here’s another:

“And when he had taken the scroll, the four living creatures and the twenty-four elders fell down before the Lamb, each holding a harp, and golden bowls full of incense, which are the prayers of the saints.” (Revelation 5:8)

A smart Alec once remarked to me, “praying to Jesus is not found in the Bible”. My answer, “it is”.

Since we can only pray to God and the Bible has at least one instance of direct prayer to Lord Jesus Christ, it further proves (just like worship – indirectly though) that Lord Jesus Christ is God in Flesh.

Here’s that verse (uttered by the very first martyr of Christianity itself – I.e. Stephen’s last words was a ‘prayer to Lord Jesus’; if that’s wrong and he shouldn’t have ‘prayed to Lord Jesus’,… Pssst, ‘heaven wouldn’t have opened’ & ‘the Bible doesn’t correct that act as false’ — I think Stephen knows doctrine perfectly which lead him to pray to Lord Jesus Christ):

(Acts 7:59 in three versions below):

International Standard Version
As they continued to stone Stephen, he kept praying, “Lord Jesus, receive my spirit!”

NET Bible
They continued to stone Stephen while he prayed, “Lord Jesus, receive my spirit!”

Aramaic Bible in Plain English
And they were stoning Estephanos as he prayed and said, “Our Lord Yeshua, accept my spirit!”

P/s 2:

Simple rule for those of us who call ourselves as Christians:

“Don’t follow the opinions of men. If it’s found in the Bible with respect to the New Testament Covenant, do it. If it’s not, don’t waste your time obeying the false teachings of men”.

Remember, “no verses, no talk” because then it’s just an opinion of men and not the Word of God (verses).