"No one who puts a hand to the plow and looks back is fit for service in the kingdom of God." What did Christ mean? Be willing to leave all?

He is saying that if you commit yourself to serve God but look back in regret at the life you are leaving behind you aren't fully committed in the first place. It is very similar to the warning the angel gave Lot and his family when they fled Sodom before God destroyed it. Because Lots wife looked back she was turned to a pillar of salt.

IF U WANT TO SERVE YHVH GOD, BETTER BE SURE THAT IS WHAT U WANT before u start out.

We too are living at a time when it is vital that we do not look back in a figurative sense. Jesus emphasized this point when he answered a man who had asked if he could return to his family to say good-bye before becoming a disciple. Jesus said: “No man that has put his hand to a plow and looks at the things behind is well fitted for the kingdom of God.” (Luke 9:62) Was Jesus being harsh or unreasonable in answering this way? No, for he knew that the man’s request was merely an excuse to evade responsibility. Jesus described such procrastination as looking at “the things behind.” Does it matter whether the person plowing looks momentarily at what is behind or actually puts down the plow and turns around? Either way he is distracted from what he should be doing, and his work may be affected negatively.

The fact of the matter: It means that if you are saved meaning truly saved you will not backslide and look back at the world, you will only do God will and the will of God is to be obedient to God holy word the bible and too spread God the Lord Jesus Christ our Lord and Savior holy word the bible throughout the entire world so that some might become saved.

Not necessarily "leave it all", but more like "don't get distracted".

In order to make straight furrows, a plowman could not be distracted by what was behind him. Similarly, a Christian cannot allow himself to be distracted by the things that he left behind in the world (things that are immoral).​ (Philippians 3:13)


When we experience difficulties, it is easy to long for ‘the good old days,’ perhaps the days before we became Christians.


If we were to dwell on such thoughts, we could be tempted to return to our former way of life. How much better it is to count our present blessings and to focus on future joys under God's kingdom!​ (2 Corinthians 4:16-18.)


Here's a short video explaining what God's kingdom is:
https://www.jw.org/en/library/books/good-news-from-god/what-is-gods-kingdom/video-kingdom-come/

We have to put god’s word first in our lives, and we can’t be longing, or focusing on the things of this world. We have to focus intently on our service to him, and keep going in that direction.

It is about "Count the cost", Luke 14:28-30.

Possibility #1
The idea that one who becomes Christian
should not "look back" (long for) their previous, sinful, non-Christian life.
Anyone who does so is not fit for the Kingdom of God.


Possibility #2
A reference to Elijah and Elisha in the Old Testament.

In the Old Testament, the prophet Elijah came to Elisha while Elisha was plowing the family fields, threw his mantle over Elisha's shoulders (offering Elisha the opportunity to follow him as his apprentice, so to speak). Elijah continued walking.

Elisha ran after Elijah and entreated, "Let me say goodbye to my mother and father first and then I will follow you."

Elijah replied (paraphrased): "Just go back."

Elisha, apparently stung, in fact did return
and detached the oxen from the plow
and sacrificed the oxen using the wood of the plow to make the sacrificial fire
(all, apparently, to prove to Elijah that he chose to dissolve any familial attachment)
and then followed Elijah, who at that point accepted him.

https://www.biblegateway.com/passage/?search=1ki+19%3A19-21&version=NASB;NRSV

Elisha became the most prolific miracle-worker of the Old Testament. Indeed: many of his miracles were duplicated by only one other Bible person...Jesus.
- the feeding of a crowd with one portion of food
- resurrection of a boy [I believe: the first recorded case of primitive CPR]
- healing leprosy (more precisely: "skin disease")
etc.

Seek the Kingdom of God without regret.

When on a mission from God, finish what you start, for God is with you.