This is what I believe the meaning is
I believe a person who does not make jesus words the foundation of His life may have believed jesus is God but He didn't find jesus as way truth and life. If he does not for example believe stealing is wrong he does not believe jesus is God by his heart. He therefore believes jesus is God for other reasons that He is not in reality. Therefore he believes jesus is God but not the God we should worship
Jesus said there are branches in Him which do not bear fruit. They are not those about whom jesus said you shall know the truth and the truth shall set you free. One person could have believed before stealing is wrong and then justified it. That is less obvious but he lost his faith. Jesus said he who has will be given more but he who has not even what he seems to have will be taken away from him. That means you can't expect to have grace as you did when you were a babe in Christ if you stop growing in knowledge or maintaining the truth in your heart or magnifying it so increasing your faith. A believer therefore may have commited many sins but he has a sensitive conscience and so God gives Him grace because he is a babes but an older believer must grow in knowledge of truth to remove the tares that are planted to take away his conscience that he may have faith.
If you are living in habitual sin you have not found the way which leads to life yet. You are not walking according to the Spirit but the flesh
My purpose of this post is to say I think I could be wrong that jesus did not intend the way to heaven being burdensome and for you to save yourself. You only have to make much effort when you have quenched the Holy Spirit. It is difficult because you have to strive to never quench the Spirit to be safe you have to fear your oil runs out when you need it most
I have not found the way yet. My point is it is difficult but not burdensome it is the good fight we need to want to do it because it is the right thing to do. Not to prove we are good but that God is good. Therefore we are not saving our reputation but Gods in our heart and glorifying Him. We can not save our reputation we are evil.
It is difficult only for the one who does not yet love God. Perfect love casts out fear
Therefore few find that it is impossible to save themselves and find the grace for God to save them to the way of life