Just to season the discussion a bit.
First, I understand that the gospel is, and should be, simple. Elementary. Not elementary as if it is a child's thing. But elementary as a foundational thing. So I absolutely support a simple message of good news to the world.
However, resilience against the work of the enemy comes through maturity. We see this in the natural: children are not left to fend for themselves. Even nature knows to protect its little ones. And the protectors are the immature are the mature. Not only are they typically older and wiser, but they also share a physical bond: the protectors tend to be the fathers and mothers of the babes.
"..to a perfect man, to the measure of the stature of the fullness of Christ; 14 that we should no longer be children, tossed to and fro and carried about with every wind of doctrine, by the trickery of men, in the cunning craftiness of deceitful plotting, 15 but, speaking the truth in love, may grow up in all things into Him who is the head—Christ"
This is the great sin of the church: they have not focused on the maturity of the believer. The leadership is to and will be to blame for the great falling away because they did not prepare the children. They were content with numbers and the income that comes from enrollment. (As an aside: the final trap of the enemy is a financial one: you cannot buy or sell without supporting the beast. If the church leadership is already corrupted by the lure of money what chance do the people under their care have to resist the evil one during his final campaign!
Even so, regarding grace, the people are taught the child's version of grace (unmerited favor) but are not taught how to walk in it, exist in it, or how God leverages grace for our benefit.
God's grace is shown as unmerited favor, for sure, but grace is primarily how God accomplishes His work.
So, grace can be shown as a man given to new believers (like Paul)
...as the authority of Christ to keep believers from the enemy
...as the goodness of God shown in His plan for men
...as the right necessary to receive good things from God
etc.
The church, by and large, has restricted grace to the salvation to go to heaven message simply because that's all they know how to teach. But it is much more. And knowing the more is a requirement for growing up into Christ.