This has been a hotly debated subject for so long in the body of christ probably beginning with CALVINISM verses ARMINIANISM which each has five seemingly conflicting points namely:
ARMINIAN : 1.Free-Will or Human Ability 2.Conditional Election 3. Universal Redemption 4. The Holy Spirit Can Be Effectually Resisted 5. Falling from Grace

CALVINISM: 1.Total Inability or Total Depravity 2. Unconditional Election 3. Particular Redemption 4.The Efficacious Call of the Spirit or Irresistible Grace 5. Perseverance of the Saints

So as you can see basically Arminian through certain scriptures believed that everyone had a freewill that determined their election, that all could be saved but we can resist the Holy spirit
call to salvation and even if answered we can fall from grace. Calvin believed the opposite he believed we are incapable of making a choice because we are so depraved therefore God has elected some to be saved and redeemed. We cannot resist the call of the holy spirit if we are elected and if saved you can never Fall or be lost.

So the term once saved always saved is a calvinistic term which is not a bible terminology. I Pastor Peter believe the truth is between the two conflicting arguments IE we are incapable of saving ourselves but we can exercise our will to choose. Many are called few are chosen the chosen being those who answer the call of whosoever will, Gods grace is irresistible but can be resisted and we can fall from grace but God is able to keep us from falling.

In answer to your question, I believe those people in matthew 7 thought they could be saved by good works without relationship and this is the danger Jesus said I NEVER KNEW YOU. The way we can ensure he does not say that to us is to live in a real, living relationship with your saviour and do not good works TO BE SAVED but because YOU ARE SAVED !