We want to not only have JOY in this life, but JOY in the next life. That is why the topic of Salvation is so important. To comprehend how Salvation is by JOY, one must understand the nature and environment of Heaven.
First, "God is a spirit":
24 God is a Spirit: and they that worship him must worship him in spirit and in truth (John 4).
Second, Heaven (where God dwells) is a spirit-based environment:
50 Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption (1 Corinthians 15).
Even after we are resurrected, our resurrected bodies will be spirit-dominated bodies and for that reason able to exist in Heaven:
44 It is sown a natural body; it is raised a spiritual body. There is a natural body, and there is a spiritual body (1 Corinthians 15).
In a spirit-based environment (be it Heaven or hell) our thoughts and heart are known. Right now, they are being hidden by our flesh:
12 For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known (1 Corinthians 13).
The Law of Attraction dominates there:
14 What communion hath light with darkness? (2 Corinthians 6).
5 Ye are all the children of light, and the children of the day: we are not of the night, nor of darkness (1 Thessalonians 5).
8 For ye were sometimes darkness, but now are ye light in the Lord: walk as children of light:
9 (For the fruit of the Spirit is in all goodness and righteousness and truth;)
10 Proving what is acceptable unto the Lord.
11 And have no fellowship with the unfruitful works of darkness, but rather reprove them (Ephesians 5).
Good fruit comes from good trees. The reverse is also true (figuratively speaking), good fruit is drawn towards good trees. In other words, there exists a Law of Compatibility in the spirit world, whether it be Heaven or hell. Sadhu Sundar Singh (1889-1929) observed this when he was permitted to see what happened to the souls of people after they died:
Sometimes we experience a sudden sensation of spiritual peace or pain, something that comes to us from the spiritual world without any thought or desire of our own. This sensation is the beginning of heaven or hell. It may recur and vary, but gradually one or the other prevails according to our spiritual orientation. By repeatedly fostering one or the other, we determine our spiritual destiny. So the foundations of heaven or hell are laid within our hearts long before we pass on to the next world. When our life in this world ends, we enter into whichever spiritual state our desires or passions have prepared." (Sadhu Sundar Singh, Wisdom of the Sadhu, Teachings of Sundar Singh, Ed. Kim Comer, 2007, New York: Plough Publishing, 165; emphasis mine.)
Continuing on ...
Then I saw another man enter the world of spirits. He had led an evil life. When the angels and saints went to help him, he cursed and reviled them, saying: 'God is altogether unjust. He has prepared heaven for such flattering slaves as you and then casts the rest of us into hell. And you call him Love!' Just then the magnificent voice of a high angel called out, saying, 'God gives this man permission to enter heaven.' Eagerly the man stepped forward. When he reached the door of heaven, however, and saw the holy, resplendent palace with all its glorious inhabitants, he began to feel uneasy. The angels encouraged him to go a little further to see the Master sitting on his throne. But when he did, the light of the Sun of Righteousness revealed the impurity of his sin-defiled life. He turned away in an agony of self-loathing and fled in such haste, that he cast himself headlong into the bottomless pit.
Then I heard the voice of the Master saying: "Look, my dear child! I forbid no one to enter my kingdom. No one forbade this man, nor did anyone ask him to leave. It was he, with his impure life, that fled this holy place. Except you be born of the spirit, you cannot see the kingdom of God. (Singh, Wisdom of the Sadhu, 167-168; emphasis mine.)
This leads us to the importance of JOY and the role it plays in Salvation. As was just established, Heaven is as much a state of being as it is a place. There is no way that a person full of darkness would be compatible with a place full of light or vice versa. Heaven is a place of brilliant light and JOY. It is not possible to simultaneously be in a state of darkness and JOY. In other words, JOY is synonymous with LIGHT.
He who is born of light is born of God. JOY is born of light; therefore, he who is born of JOY is born of God and will be brought into Heaven like on a tractor beam from Star Trek. JOY is an attribute and/or disposition of God that knows no bounds. It is not based on language or knowledge. Remember, children are born this way. It is not something we learn, it is something that our soul retains from its time there in Heaven, and because Heaven is inside us.
If we protect our natural, inborn state of JOY, we are protecting the Kingdom of God within us:
21 Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you (Luke 17).
We lose our JOY at the same rate we lose our conscience. When we do what our conscience dictates, we have JOY; we feel good. It's when we compromise our conscience that we experience GUILT, which is a great destroyer of JOY. Paul said we will be judged by the state of our conscience or, in other words, by the state of our JOY:
14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another (Romans 2).
To confirm the truth of this point I offer the following verses:
2 Behold, God is my salvation; I will trust, and not be afraid: for the Lord Jehovah is my strength and my song; he also is become my salvation.
3 Therefore WITH JOY shall ye draw water out of the wells of salvation (Isaiah 12).
"God is our salvation" and the way that we draw out our salvation is by JOY.
3 Verily I say unto you, Except ye be converted, and become as little children [JOY-filled], ye shall not enter into the kingdom of heaven (Matthew 18).
14 But Jesus said, Suffer little children, and forbid them not, to come unto me: for of such is the kingdom of heaven [full of JOY] (Matthew 19).
Looking through the prism of JOY at our God and His domain will secure more souls in His Kingdom than anything else.