ACC ~ Romans 14 v23

ALL THAT IS NOT OF FAITH IS SIN. BASIL: If “all that is not of faith is sin, and faith comes by hearing, and hearing by the Word of God,” then everything outside holy Scripture, not being of faith, is sin. THE MORALS 80.22.

EITHER/OR. GREGORY OF NYSSA: Every word or deed or thought which does not look to Christ looks completely to the adversary of Christ. For it is not possible for what is outside of light or life not to be completely in darkness or death. … The person outside of Christ rejects him by what he thinks, does or says. ON PERFECTION.

RESPECT CONSCIENCE. AMBROSIASTER: It is true that if someone thinks it wrong to eat but does so anyway, he is condemned. For he makes himself guilty when he does what he thinks he ought not to.

If someone acts against his better judgment in a matter of conscience, then Paul says that it is a sin.COMMENTARY ON PAUL’S EPISTLES.

CHRYSOSTOM: Once again, Paul shows what great harm people do if they force people to do things which go against their conscience. When a person does not feel sure or believe that something is clean, how can he do other than commit sin? HOMILIES ON ROMANS 26.

ONE WHO INCREASES DOUBT IS CONDEMNED. [PSEUDO-]CONSTANTIUS: If one person has doubts about another and says: “If this man has eaten he has obviously been condemned,” then it is clear that he has no faith. … For it is not of faith if you condemn someone else because while you are fasting he is eating. THE HOLY LETTER OF ST. PAUL TO THE ROMANS.

PELAGIUS: Whatever destroys another is not of faith and is therefore sin. PELAGIUS’S COMMENTARY ON ROMANS.