Most assuredly, I say to you, the hour is coming, and now is, when the dead will hear the voice of the Son of God; and those who hear will live. (John 5:25)
This verse is from the Gospel of John who wrote to the whole world, in order that we might believe Christ was the Son of God (John 20:31). Jesus had, prior to this utterance, healed a sick man at the Pool of Bethesda, on a Sabbath, by instructing him to take up his bed and to walk – despite the Pharisaical law that prevented both healing and carrying a bed on the Sabbath. This led to active persecution of Jesus by the Jews to the extent they wanted to kill him (John 5:16). His marvelous rebuttal argues that because his Father had been working up to now, so shall he. And still worse, he related the man’s illness to sin, saying” "See, you have been made well. Sin no more, lest a worse thing come upon you." (John 5:14). In the eyes of the Pharisees, Jesus had made himself equal with God – which he was, but his work was deemed blasphemy by the Pharisees.
After this episode Jesus rebuts the Pharisaical assertions in verses 19 – 29. I will not dwell on this rebuttal, but rather focus on the context in order to understand verse 25, the verse in question. In this discourse he makes two major assertions:
These two assertions are linked. The key in understanding these relies on the fact that all humans are born into darkness due to our inherited sin nature – that is, we all are born spiritually dead. And furthermore, there is nothing any human can do to make this right. Jesus points out in Chapter 3 of John the long held fact that only by believing on the Lord Jesus Christ is one saved.
For this reason the gospel was preached also to those who are dead, that they might be judged according to men in the flesh, but live according to God in the spirit. (1 Peter 4:6)
To understand John 5:25 the key is to understand that all people are spiritually dead until they are born again. Two verses (among many) to illustrate this:
And you He made alive, who were dead in trespasses and sins (Ephesians 2:1)
Even when we were dead in trespasses, made us alive together with Christ (by grace you have been saved) (Ephesians 2:5)
One can speak of two deaths – physical death which occurs at the end of our life, and spiritual death, the state we all are when born. On believing in Jesus Christ, the Holy Spirit brings us to life (the term ‘quicken’ is used in old English). To believe we need to hear – hear the gospel. Thus salvation requires two elements: 1) hearing the word of God and 2) believing the word of God.
So then faith comes by hearing, and hearing by the word of God. (Romans 10:17)
Verse 25 of John 5 therefore is a summary of salvation: soon after this saying, Jesus died for our sins, and from that point onward, the spiritually dead would be able to hear the voice of the Son of God, through the Holy Spirit, and those that hear will be saved. Note that hearing is more than just hearing a sound – here, hearing is the assimilation of the force of the words – indeed hearing Christ Jesus, which leads to a person submitting their hearts to Christ – and is enabled by the Holy Spirit – is an act of faith.
The consequence of a quickened soul, a trusting heart and a regenerated soul is everlasting life, obtain through faith in Christ. This is possible because the sin which separated us from God has been paid in full by the sacrifice of Christ shed at Calvary – that person has been justified:
Most assuredly, I say to you, he who hears My word and believes in Him who sent Me has everlasting life, and shall not come into judgment, but has passed from death into life. (John 5:24)
Much more then, having now been justified by His blood, we shall be saved from wrath through Him. (Romans 5:9)
 This was an extension of Nehemiah 13:15 & Jeremiah 17:21 were burdens could not be carried on the Sabbath – i.e. transport of your weekly produce could not occur on the Sabbat.