How do the reformed theologians understand Hebrews 3:14? How do the reformed theologians understand 


  Hebrews 3:14 For we have come to share in Christ, if indeed we hold
  our original confidence firm to the end. ( ESV )


?

Does it not imply that we can lose salvation?
John Piper comments on the passage here: https://www.desiringgod.org/messages/do-not-harden-your-heart-in-the-day-of-trouble

He contends that the "if" clause means something other than what you might think at first. Instead of holding your assurance (persevering in faith) until the end to become a partaker in Christ, you prove that you already are a partaker in Christ by persevering. Here is a quote:


  Now jump ahead to verse 14 to confirm that this is the way the writer
  is thinking. In verse 14 we have an "if" statement very much like the
  one in verse 6: "We have become partakers of Christ, if we hold fast
  the beginning of our assurance firm until the end." Being "partakers
  of Christ" in verse 14 is virtually the same as being "partakers of
  the heavenly calling" in verse 1. And both are the same as "being
  God's house" in verse 6.
  
  But notice the wording carefully here in verse 14, because it is a
  strong confirmation that we are on the right track. It says, "We have
  become partakers of Christ, if we hold our assurance to the end." The
  condition is future: "If we hold fast assurance to the end." But the
  effect of the condition relates to the past: "We have become partakers
  of Christ." So it's clear that the point here is not: hold fast to
  your assurance in order to become in the future a partaker of Christ.
  The point is: hold fast to your assurance in order to show (prove,
  evidence, demonstrate) that you are a partaker of Christ.


This article makes reference to Spurgeon, whose sermon on the topic is dense and hard to pull a cogent quote out of. The article, however, does make the point concisely.
https://www.neverthirsty.org/bible-studies/book-of-hebrews/warning-part-2/

A brief quote:


  Hebrews 3:14 says that those who become Christians will continue until
  the end. The Greek tense of “have become” is a perfect. This means
  that real Christians start as Christians and continue being
  Christians. Real Christians continue in the faith – “if we hold fast.”
  Did the Spirit change His mind when He came to Hebrews 6:4? Is it
  possible to start as a real Christian and then not continue? Hebrews
  4:10 also says that Christians rest from working for their salvation
  just as God rested. The message in Hebrews 3 and 4 is that if you are
  real, you are real until the end. If we do not hold fast, we were
  never Christians from the start.


The Spurgeon sermon on this verse: https://www.spurgeongems.org/vols16-18/chs1042.pdf

The following paper by Dan Sanders discusses Jonathan Edwards' views on assurance:

https://churchsociety.org/docs/churchman/126/Cman_126_2_Saunders.pdf

Here is a quote (not of Edwards, but of the author):


  Finally, Hebrews 3:14 uses βεβαίαν with ὑποστάσεως. 29 We hold firm
  to our ‘first confidence’, or rather the ‘beginning of the reality’,
  referring directly to Christ (3:14a) or possibly to ‘the hope’ of the
  parallel confidence (3:6).30 We share in Christ and hold firm to
  Christ—the ὑπόστασις, the divine reality—by reflecting on Jesus
  (3:1) with a faithful heart not hardened by sin. Therefore, God is the
  author of our growth, richly providing everything needed for
  godliness. Jesus strengthens believers as the gospel is strengthened
  in the church. As we hold firm to Christ and our hope, we share in
  Christ. Hence, assurance, already existing in faith, grows as we do
  works of faith.