From the Old Testament: Fear God and depart from all evil (Proverbs 3:7).
From the New Testament: For I have received of the Lord that which also I delivered unto you, that the Lord Jesus the same night in which he was betrayed took bread: And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me. After the same manner also he took the cup, when he had supped, saying, this cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me. For as often as ye eat this bread, and drink this cup, ye do shew the Lord’s death till he come. Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord. But let a man examine himself, and so let him eat of that bread, and drink of that cup. For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord’s body” (1 Cor. 11:23-29, Italics added).
From a Modern Saint: “Set this as the goal of your preparation for Holy Communion, so that at the end of this preparation the fear of God will dwell in you and a firm intention to avoid every evil will take root, even if you should lose everything, including your life, in order to acquire this. Do not limit yourself to an external routine of preparation alone, but focus in particular on yourself. Enter within yourself and examine your views, as to whether they are in agreement in all things with the true word of God. Examine your inclinations and dispositions, as to whether they are what the Lord demands of you in the Gospels. Examine your whole life, as to whether it agrees with God’s commandments in every way. Bemoan and hate whatever is offensive to God and set in your mind never to do it again. If you do this, you will be most wise.” — St. Theophan the Recluse (1815-1894)