Those who "depart from the faith" were never genuinely saved to begin with. Such "nominal" Christians had an outward, association with the church or Christian community rather than true, regenerating faith. Just like in 1 John 2:19, we read - They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us.
"The faith" refers to the objective body of Christian truth and doctrine, rather than personal saving faith in Christ. To "depart from the faith" means to abandon or turn away from sound Christian teaching to follow false, deceitful spirits and demonic doctrines (such as forbidding marriage and requiring abstinence from foods that God created to be received with thanksgiving). This confirms that such folks were never firmly rooted and established in the faith.
Yes, and the scripture confirms this regarding 'the faith'
They depart from the external religion, church, the faith, never having been converted by the Spirit on the inside into the New Man
Ephesians 4
The New Man
17 This I say, therefore, and testify in the Lord, that you should no longer walk as [
f]the rest of the Gentiles walk, in the futility of their mind, 18 having their understanding darkened, being alienated from the life of God, because of the ignorance that is in them, because of the blindness of their heart; 19 who, being past feeling, have given themselves over to lewdness, to work all uncleanness with greediness.
20 But you have not so learned Christ, 21
if indeed you have heard Him and have been taught by Him, as the truth is in Jesus: 22 that you put off, concerning your former conduct, the old man which grows corrupt according to the deceitful lusts, 23 and be renewed in the spirit of your mind, 24
and that you put on the new man which was created according to God, in true righteousness and holiness.
Greeting to the Called - Jude, a bondservant of Jesus Christ, and brother of James, To those who are called, sanctified by God the Father, and preserved in Jesus Christ: Mercy, peace, and love be multiplied to you. Contend for the Faith - Beloved, while I was very diligent to write to you...
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Greeting to the Called
1 Jude, a bondservant of Jesus Christ, and brother of James,
To those who are called, [
a]sanctified by God the Father, and preserved in Jesus Christ:
2 Mercy, peace, and love be multiplied to you.
Contend for the Faith
3
Beloved, while I was very diligent to write to you concerning our common salvation, I found it necessary to write to you exhorting you to contend earnestly for the faith which was once for all delivered to the saints. 4
For certain men have crept in unnoticed, who long ago were marked out for this condemnation, ungodly men, who turn the grace of our God into lewdness and deny the only Lord [b]God and our Lord Jesus Christ.
Old and New Apostates
5
But I want to remind you, though you once knew this, that the Lord, having saved the people out of the land of Egypt, afterward destroyed those who did not believe. 6 And the angels who did not keep their [
c]proper domain, but left their own abode, He has reserved in everlasting chains under darkness for the judgment of the great day; 7 as Sodom and Gomorrah, and the cities around them in a similar manner to these, having given themselves over to sexual immorality and gone after strange flesh, are set forth as an example, suffering the [
d]vengeance of eternal fire.
Hebrews 10
32 But recall the former days in which, after you were [
h]illuminated, you endured a great struggle with sufferings: 33 partly while you were made a spectacle both by reproaches and tribulations, and partly while you became companions of those who were so treated; 34 for you had compassion on [
i]me in my chains, and joyfully accepted the plundering of your [
j]goods, knowing that you have a better and an enduring possession for yourselves [
k]in heaven. 35 Therefore do not cast away your confidence, which has great reward. 36 For you have need of endurance, so that after you have done the will of God, you may receive the promise:
37 “For yet a little while,
And He[
l] who is coming will come and will not [
m]tarry.
38
Now the[n] just shall live by faith;
But if anyone draws back,
My soul has no pleasure in him.”
39
But we are not of those who draw back to [o]perdition, but of those who believe to the saving of the soul.