The doctrine of "once saved always saved" was introduced into Christian beliefs by satan, because who else but satan wants Christians to avoid doing God's work. Most people go along with these doctrine because it is safe and easy, there is no fear they will go to Hell so they adopt it. Two scriptures said by Jesus calls out these types of beliefs:
Matthew 7:13 Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat:
Matthew 7:14 Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it.
Matthew 16:27 For the Son of man shall come in the glory of his Father with his angels; and then he shall reward every man according to his works.
Matthew 16:28 Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.
Jesus is speaking directly in a large part to those he knows at the end will be under the false doctrine of not needing works, and thinking they have a free pass to Heaven. Matthew 16 directly addresses this false doctrine, "Son of man shall come....he shall reward every man according to his works...there be some standing here, which shall not taste of death, till they see the Son of man". This goes back to Matthew 7 telling us is "narrow is the way, which leadeth unto life, and few there be that find it". What is "narrow" or hard about "I believe Jesus is the Son of God!" That is great! But...it brings us to:
James 2:19 Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
James 2:20 But wilt thou know, O vain man, that faith without works is dead?
But there will be the people set in traditions of man, who will bring up the one and only verse "we are saved by grace, lest any man boast". The keyword here is "BOAST", not that Jesus is a liar and James is a liar. The point is just to humble people, that doing works and trying to get to Heaven WITHOUT God's grace and permission is not going to happen, you need both. But the most ironic thing about people quoting Ephesians is that they are saying "Paul is the true prophet of God, Jesus Christ was a liar when he keeps talking about works". If Jesus says you need works and Paul says you do not need works, who as CHRIStians are we suppose to listen too? Jesus. So if Jesus is right and knows more about his Father's law then Paul, then the next step is to figure out what Paul was trying to tell us since he was not calling Jesus a liar. Again, the keyword "boast" comes in, and Christians not trying to enter Heaven without God's grace.
Ephesians 2:8 For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:
Ephesians 2:9 Not of works, lest any man should boast.