Obviously if you are Abraham, Issac or Jacob, you will feel very much secure in your salvation.
However, going down the line, the security gets lesser and lesser. King David was apparently very worried that God will take the Holy Spirit away from him after he committed adultery with Bathsheba.
Psalm 51:11
As Paul revealed later on, David saw in the future a blessed group of believers, which is us under the grace dispensation, whose salvation is so secure that God will never impute sins to our account. Romans 4
6 Even as David also describeth the blessedness of the man, unto whom God imputeth righteousness without works,
7 Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered.
8 Blessed is the man to whom the Lord will not impute sin.