Once saved always saved is the heart of the gospel! and the epistles. If you have no seen the verses, you have not read the NT much then. They are everywhere!
As for Luke- instead of making up things the bible does not declare, we should just simply accept what teh word does say about people who start with us and then leave:
1 John 2:19
They went out from
us, but they
were not of us; for if they had been
of us, they would no doubt have continued with
us: but they went out, that they might be made manifest that they
were not all
of us.
Mt. 13:
24 Another parable put he forth unto them, saying, The kingdom of heaven is likened unto a man which sowed good seed in his field:
25 But while men slept, his enemy came and sowed tares among the wheat, and went his way.
26 But when the blade was sprung up, and brought forth fruit, then appeared the tares also.
27 So the servants of the householder came and said unto him, Sir, didst not thou sow good seed in thy field? from whence then hath it tares?
28 He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up?
29 But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them.
30 Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.
Matthew 7:21-23 King James Version (KJV)
21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
In Hebrews the Lord reminds us He will never leave us or forsake us! So if a child of God turns their back on Jesus- Jesus still is with them. They may be disobedient children, but they are still children of god!