The best way to make sure that someone never understands the Gospel is to make them good.
I think this is what Jesus means when he tells the Pharisees that they go across sea and land to make a single convert and then make him "twice the son of hell" as they themselves are. There was a certain zeal to their activity; perhaps they were driven by a passion for their message. They may have appeared enthusiastic. Yet, they functionally shut the door to the kingdom of heaven in people's face. Why?
Because they taught people how to be saved without Jesus. They made people good. They told them how they could behave better, have a better life, be more religious, and please God on their own. Once they believed that they could please God with their own efforts, they were in double trouble. It's one thing to be ignorant of the fact that you have a problem (in this case, God's displeasure towards your best efforts); it's another thing entirely to believe that you've solved the problem on your own.
It's bad to have cancer and not know it. It's worse to have cancer but convince yourself everything is okay. In the case of the former, you might be open to the real remedy once the problem is revealed. In the case of the latter, you don't even think you need a remedy.
The way we do this in the world of American Christianity is giving people just enough Jesus that they don't ever bother to look for the real thing. We give them a vaccine. They are inoculated. And we do it by making them good.
Growing up, we were made good through religious activity. We had solid theology and doctrine, we just didn't have much of Jesus. We assumed Jesus. We could quote answer #1 from the catechism (at least the first part), we just didn't know how to actually get the comfort that we said we had (the second part of the answer). (Side note: question #2, which no one memorizes, also directs us towards the answer...)
Nowadays the pendulum has swung in the other direction. We no longer address doctrine, theology, or that sort of deep, boring, and confusing stuff. We just "follow Jesus". We're not entirely sure which Jesus, or what Jesus believed, or what he taught, or any of that confusing stuff. We do know how to be better parents, better lovers, and better employees, though, so it can't be all bad. Anything other than that we can just sort of make up as we go.
The end result is the same in both categories. It's either something we do or something we know that makes us okay with God. Either we know a lot about him, or we follow him. Unfortunately, neither is ultimately sufficient.
The heart of Christianity is putting our total confidence in Christ's work rather than our own; it's understanding God's absolute and one-way love towards sinners like us. It means admitting that my best isn't good enough. I can't earn God's acceptance–but I don't have to, because Jesus already has. God gives it to me, free of charge. That's grace. And it's the only way in to the kingdom of heaven.
“But woe to you, scribes and Pharisees, hypocrites! For you shut the kingdom of heaven in people’s faces. For you neither enter yourselves nor allow those who would enter to go in. Woe to you, scribes and Pharisees, hypocrites! For you travel across sea and land to make a single proselyte, and when he becomes a proselyte, you make him twice as much a child of hell as yourselves.”
“Q. What is your only comfort
in life and in death?
A. That I am not my own,
but belong—
body and soul,
in life and in death—
to my faithful Savior, Jesus Christ.
He has fully paid for all my sins with his precious blood,
and has set me free from the tyranny of the devil.
He also watches over me in such a way
that not a hair can fall from my head
without the will of my Father in heaven;
in fact, all things must work together for my salvation.
Because I belong to him,
Christ, by his Holy Spirit,
assures me of eternal life
and makes me wholeheartedly willing and ready
from now on to live for him.”
“Q. What must you know to
live and die in the joy of this comfort?
A. Three things:
first, how great my sin and misery are;
second, how I am set free from all my sins and misery;
third, how I am to thank God for such deliverance.”