“have been made partakers of the Holy Spirit”
If that does not describe a Christian, what does?
“12 For just as the body is one and has many members, and all the members of the body, though many, are one body, so it is with Christ. 13 For in one Spirit we were all baptized into one body—Jews or Greeks, slaves or free—and all were made to drink of one Spirit.”
While I believe Hebrews 6 is incontrovertible, it is not the only example. Indeed, there are many:
“Such warnings cannot be easily dismissed, and the Christian theologian should be diligent in giving full consideration to the entirety of the book of Galatians in formulating his doctrine of continuance in salvation. In particular, hear once again Paul’s words as he winds down his argument: “Do not be deceived: God cannot be mocked. A man reaps what he sows. Whoever sows to please their flesh, from the flesh will reap destruction; whoever sows to please the Spirit, from the Spirit will reap eternal life. Let us not become weary in doing good, for at the proper time we will reap a harvest if we do not give up” (6:7-9).”
Also http://evangelicalarminians.org/apostasy-and-perseverence/
No one simply reading the New Testament would conclude no Christian can leave the Lord. It is filled with warnings about falling away, and full of encouragement to stay the course to the end. It takes an external theology imposed on top of the scripture to make the obvious go missing.
.
>> “if we do not give up” <<
Which means that at some point we have to have started.
.