There are some Christian groups who believe that all religions are inherently good and that a good person who rejects Jesus Christ will still go to Heaven. According to them, if good people can go to Heaven regardless of what their religion is, then do we still have to tell them about Jesus Christ or we can just leave them alone to worship their own god, gods or goddesses?
Because Jesus is that important to you, and you want him known to all the world.
From the linked question, I am assuming you are asking on the perspective of Swedenborgianism, which is forms the doctrinal foundation of the New Church
According to Dream Prophecy
New Church tends not to evangelize, and respects people's free will in thinking on their own. Nor does it condemn other religions to hell.
That said, according to NewChurch.org historically they did evangelize, in the form of spreading awareness.
In the latter part of the century, the ministry almost doubled in size and an expansion of evangelization work was initiated to spread awareness of the new teachings and further grow the church.
According to Swedenborg.org
Salvation is an inner experience rather than acceptance of a particular doctrine.
The Second Coming is an active process that is happening within us and the world. As we increase compassion, integrity, understanding, and healing in our lives, we are helping God create a "new Heaven and a new Earth."
From Arcana Coelestia 6488
A discussion about predestination once took place and many of the spirits, owing to the chief ideas they had adopted in the world, were of the opinion that some people have been predestined to heaven and others to hell. But I heard a response to this from heaven, which was that no one has ever been predestined to hell, but that all have been predestined to eternal life
From the above, it seems that in a Swedenborgianism perspective it is not necessary to evangelize concerning salvation.
From Swedenborg Study
The Lord Jesus Christ is our God. There is no other. To Him we owe all that is good and all that is true. All power in heaven and on earth is His. To Him alone should we pray. To Him alone should be our worship, our love, and the service of our lives.
- To make known that the Lord is here
The agencies for this Proclamation are many,-organization, the priesthood, preaching and public worship, printing and publishing the books of the new Revelation, and other modes of evangelization, that men may know that the Lord is here.
Even if you believe that Salvation is for all, but you believe that Jesus is the only one true God, you will still want others to know Him as well.
Here is a response from the perspective of the theology of Emanuel Swedenborg (1688-1772) and sort of from the perspective of the New Church, or Swedenborgian, churches that accept his theology.
I say "sort of" because although there were a number of active and reasonably successful Swedenborgian missionaries in the 19th century, that activity fell off in the 20th century. Various Swedenborgian organizations are making some efforts at evangelization today. However, it has generally not been a strong and integral part of the life of most Swedenborgian churches for the past century or so. The reasons for this go beyond the scope of the question, so I will not delve into them here. However, the belief that people of all religions can be saved is only one of those reasons, and probably not the primary one.
Swedenborg himself made consistent efforts to publicize and evangelize the theology he was presenting to the world. He did this primarily by indefatigably writing about and publishing that theology during the final third of his lifetime, and sending copies of his books to religious leaders, universities, and seminaries throughout Europe. He also commissioned others to pass the books on to people whom he thought would be interested in the teachings they articulated.
In line with Swedenborg's own practice, the most consistent evangelization efforts by Swedenborgians over the years have been in the realm of publishing and the printed word. The number of books published by Swedenborgians and Swedenborgian organizations over the past couple of centuries is all out of proportion to the small numbers of the faithful.
A technical answer to the question
Now to answer more specifically. The question is:
If good people can go to Heaven regardless of what their religion is, then do we still have to tell them about Jesus Christ or we can just leave them alone to worship their own god, gods or goddesses?
To get technical for a moment, it all depends what you mean by "have to."
Do Swedenborgians have to evangelize or they'll go to hell? No. Jesus' primary commandment is to love God above all and love our neighbor as ourselves (Matthew 22:34-39). If we follow that commandment, we will be saved even if we don't evangelize.
However, Jesus does command us to evangelize:
All power in heaven and on earth has been given to me. Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to obey everything that I have commanded you. And remember, I am with you always, to the end of the age. (Matthew 28:18-20)
That commandment applies to Swedenborgian Christians just as much as it does to every other variety of Christian.
So if "do we still have to" means "are we still commanded to," then the answer is yes, Swedenborgians are commanded to tell others about Jesus Christ just as all other Christians are.
The real answer to the question
Now, I could stop right here, and it would be a technically correct but rather unsatisfying answer.
The underlying question seems to be why would those who believe people of all religions can be saved feel that they have to tell others about Jesus Christ?
To answer this question, it is necessary to take a closer look at the Great Commission itself, as quoted above from Matthew 28:18-20.
Notice that Jesus does not say, "Everyone who doesn't believe in me is going to hell. Go therefore and make disciples of all the nations . . . ." No, he says:
All power in heaven and on earth has been given to me. Go therefore and make disciples of all the nations . . . .
In other words, the reason Jesus himself gives for evangelizing is not that otherwise people will go to hell, as is believed in many Christian denominations. Rather, it is, in essence, that he is God, and he wants people to know that.
Anyone who has all power in heaven and in earth is God, because only God has all power.
Swedenborgians follow Swedenborg's theology in believing that Jesus is not some second Person of a divine Trinity, but rather is the one God of heaven and earth, whose divine soul, or divine love, is called "the Father" in the Bible, and whose power flowing out into the world is called "the Holy Spirit" in the Bible. (For more on this, see my article, "Who is God? Who is Jesus Christ? What about that Holy Spirit?")
Therefore, if anything, people who accept Swedenborg's theology have an even greater duty to follow the Great Commission because their theology is based on the idea that the Lord God Jesus Christ is the only God of heaven and earth, and it is their duty to spread this teaching and belief to the world.
The primary reason to evangelize from the perspective of Swedenborg's theology is exactly the same as given in the Great Commission itself: Because Jesus Christ is the one God of heaven and earth, and he commands us to tell people of all the nations about that supreme truth.
This is why followers of Swedenborg's theology have a duty to evangelize.
People are confused, hurting, and in need of repentance
However, there is another, perhaps even deeper reason for Swedenborgians to evangelize, based on exactly what Jesus told his disciples to do when he sent them out to make disciples. For example:
They went out and proclaimed that all should repent. They cast out many demons, and anointed with oil many who were sick and cured them. (Mark 6:12–13)
If we look at these activities from a deeper and more spiritual perspective, as Swedenborgians generally do, we have our marching orders for evangelization.
1. They went out and proclaimed that all should repent.
It is an integral part of Swedenborg's theology that real religion involves ceasing to do evil and learning to do good (Isaiah 1:16-17). Repentance from sins and reformation into a person who does good instead of evil is central to Christianity from a Swedenborgian perspective.
The commandment and practice of Jesus and all of his disciples to call people to repentance is therefore a key part of the reason people who follow Swedenborg's theology are to go out and evangelize. There are many people out there living evil lives that lead only to destruction. God commands us to reach as many of them as we can with a message of repentance and new life in Jesus Christ.
2. They cast out many demons.
There are many types of demons that possess people figuratively, if not literally. People struggle with evil and destructive impulses, with the effects of abuse and neglect, with the uncaring and materialistic nature of the world, and so on.
Rather than deal with all of those, I'll just deal with one: people struggle with false ideas that tear them down and damage their spiritual, emotional, and social life.
Swedenborg's theology offers a clear, comprehensive, beautiful, understandable, and loving understanding of God, spirit, and the purpose of our life here on earth. For one contemporary perspective on this, see my article, "Heaven, Regeneration, and the Meaning of Life on Earth."
Followers of Swedenborg's theology have a duty to reach out to those who are lost and confused by false doctrine and false ideas, and offer them the clear and satisfying truth offered by that theology, which Swedenborgians see as genuine Christianity.
I am the light of the world. Whoever follows me will never walk in darkness but will have the light of life. (John 8:12)
It is our duty to spread that light to all who are ready and willing to see it.
3. They anointed with oil many who were sick and cured them.
Spiritually speaking, those who are sick are those who are hurting emotionally and spiritually. What they especially need is not truth or doctrine, but the "oil" of human kindness and love.
People who are hurting and broken in spirit need to know that they are loved, that someone cares about them, that they are not alone. Those who follow Swedenborg's theology, even if some of them may not understand the doctrines of the church all that well, generally feel a calling to reach out to those who are desolate, and offer them the spiritual oil of human kindness, sympathy, and love.
From a Swedenborgian perspective, this, perhaps, is the deepest form of evangelism: spreading the love of the Lord God Jesus Christ to all who are in need of it through acts of simple human love and kindness. See Matthew 25:31-46.
Do those who follow Swedenborg's theology have to tell others about Jesus Christ? In a technical sense, no. But if they do not, they are avoiding the direct commandment of Jesus Christ himself, and are not living in the spirit of their own theology and beliefs.
In short, for many reasons integral to their own theology, those who follow the teachings of Emanuel Swedenborg are commanded and required to "make disciples of all nations" according to the Great Commission given by the Lord God Jesus Christ.
For another presentation of these and related ideas, please see my article (written in response to a similar question from a reader), "If Non-Christians can Go to Heaven, Why should Christians Evangelize?" This article deals more directly with the question of whether to evangelize people of other religions.