I think perhaps this is Hebrews 6:4-7:
Hebrews 6:4-7 (KJV)
4 For
it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
5 And have tasted the good word of God, and the powers of the world to come,
6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put
him to an open shame.
7 For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God:
There are a couple of views of this verse and the interpretations are not without disagreement. I will give you what i believe to be the meaning of the verse and this is from the Ryrie Study Bible.
(3) Others understand the passage to be a warning to genuine believers to urge them on in Christian growth and maturity. To "fall away" is impossible (since, according to this view, true believers are eternally secure), but the phrase is placed in the sentence to strengthen the warning. It is similar to saying something like this to a class of students: "It is impossible for a student, once enrolled in this course, if he turns the clock back [which cannot be done,] to start the course over. Therefore, let all students go on to deeper knowledge." In this view the phrases in
Hebrews 6:4-5 are understood to refer to the conversion experience. Notice how the words "enlightened" (
Hebrews 10:32), "taste" (
Hebrews 2:9), and "partakers" ("share,"
Hebrews 12:10) are used elsewhere in Hebrews of genuine experiences.
Ryrie Study Bible.
The reason I think this is the meaning is because I don't believe one can believe and become a Christian and then for whatever reason, loose their salvation. One of the verses that teaches this is:
John 10:25-29 (ASV)
25 Jesus answered them, I told you, and ye believe not: the works that I do in my Father's name, these bear witness of me.
26 But ye believe not, because ye are not of my sheep.
27 My sheep hear my voice, and I know them, and they follow me:
28 and I give unto them eternal life; and they shall never perish, and no one shall snatch them out of my hand.
29 My Father, who hath given
them unto me, is greater than all; and no one is able to snatch
them out of the Father's hand.
In verse 29, Christ assures the believer that no one can take him from Christ. This is all inclusive so that there is absolutely no person or entity can take the believer from Christ's hand.