First of all, Galatians 5 is about corrupting the simple powerful Gospel of Christ with legalism. The Leaven is Legalism .
Secondly, Leaven is a good thing in bread. Have you ever eaten unleavened bread? Leaven is what causes bread to rise, to grow, to expand.
This 'quality' of expansion is the main attribute of leaven.
So, to categorically say that Leaven is always Evil is wrong. It depends on the context.
Thirdly, why are we commanded to preach the Gospel to every creature?
Answer: So that many will receive Christ and become children of God . ( God is not willing that any should perish but that all should come to Repentance ).
Is Christ King of Kings and Lord of Lords now? Yes.
Has all Power in Heaven and Earth already given to Christ ? Yes.
Who does the earth belong to then?
Christ .
Who are His Stewards (caretakers of His property) in His absence? We are.
Fourthly, the person who wrote the article spreading Fear , Doubt and confusion about the parables of Jesus is in the bonds of Deception himself.
He is indirectly denying the Power of God to Reign on Earth through Spirit-filled men. (Having a form of godliness but denying the Power , from such turn away).
Jesus was talking to Jews when He said "few will find it". He was absolutely correct. There are relatively very few Jews who have accepted Christ over 2 millenium.
Thousands of Gentile people accepted Christ during the first century church. The Catholic church sterilized the Gospel message and a thousand years of darkness settled on the earth.
The reformation was the beginning of Gospel Truth coming to light. The reason the church is still weak is that many deny the HolySpirit who is the Power of God .
Jesus said without me ye can do nothing. The HolySpirit is Christ in us the Hope (expectation) of Glory .
Satan is the thief who steals the Truth kills Hope and destroys men's Dutures .
Fifthly, negative fatalistic preaching and teaching is satanically motivated.
Remember the servant with the one talent? He was afraid and went and hid it rather than investing it for increase. He was cast out for his unprofitability.
We are to occupy till He comes. That means we are to do His work by His Power in His name. We are not to sit around waiting for His return trying to identify 'true' Christians.
One last thought, we are Saved by Faith in Christ Jesus - not by our end-time theories or human explanations of Spiritual concepts that can only be revealed and understood by the HolySpirit .