That is wrong to say, because I did not get this understanding from the Catholic church.
I got this understanding by actually studying the scriptures, and letting the Holy Spirit guide me in my learning. When ever I pray to God in the name of our Lord Jesus Christ for understanding, and an answer in a area. Within 24 hours I am guided to pick up the bible and amazingly thanks to God I turn directly to the passage that answers it.
I can feel the Holy Spirit guiding my actions, and feel His conviction toward me when I do wrong bringing me to ask forgiveness of that sin.
He has lead me to multiple scriptures that speak of believers and how they can and have fallen away from God. I have gone and posted these multiple scriptures over and over again. The only debate by you and others toward what I have given is wrong context, or just a handful of the same scriptures that are misused because only half the meaning is given or placing it in a position that does not apply.
If I or somebody else show somebody 25 scriptures that show believers can fall away, and all the other side can do is keep posting the same 3 to 5 scriptures to say believers can not. I would have to go with they can, and that the few given are misused.
Common misuses;
John 3:16
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
People want to say believe only or faith only, but what does the Lord say here. He says should not, as an imperative that one can still perish depending on their belief. Then to show what believing in Him means, He gives us this scripture;
Luke 6:46-49
And why call ye me, Lord, Lord, and do not the things which I say? Whosoever cometh to me, and
heareth my sayings, and doeth them, I will shew you to whom he is like: He is like a man which built an house, and digged deep, and laid the foundation on a rock: and when the flood arose, the stream beat vehemently upon that house, and could not shake it: for it was founded upon a rock.
But he that heareth, and doeth not, is like a man that
without a foundation built an house upon the earth; against which the stream did beat vehemently, and
immediately it fell; and the ruin of that house was great.
The other misuse I see all the time is 1 John 2:19, about the they went out from us because they were not of us. Please look at verse 18 and you will see who this is referring to;
1 John 2:18
Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time.
If you think this refers to a person who accepted Christ as their Lord, and then do to a hard trial in their life turned away from God is a form of an antichrist. Then you can make it fit, but to me an antichrist is one who is against God, and their soul purpose is to corrupt the word leading some astray. A person who initially accepted Christ, but then turned their back because of a hard time or event in their life is not a antichrist to me.
Plus to show the gross misuse of this lets go see if John agrees with the Lord that one must keep and obey the Lord's commands;
1 John 2:3-6
3 We know that we have come to know him if we keep his commands.4 Whoever says, “I know him,” but does not do what he commands is a liar, and the truth is not in that person. 5 But if anyone obeys his word, love for God[a] is truly made complete in them. This is how we know we are in Him: 6 Whoever claims to live in him must live as Jesus did.