While the Law demanded fulfilling vows and forbid swearing falsely (Leviticus 19:12; Numbers 30:2-16; Deuteronomy 23:21-23), Christians must not swear at all (JUSTIN MARTYR, IRENAEUS, CLEMENT OF ALEXANDRIA, TERTULLIAN, CYPRIAN) and always speak the truth (JUSTIN MARTYR, IRENAEUS, HERMAS).
This precept is to be literally observed (ORIGEN).
Prior to martyrdom, POLYCARP was urged to “swear by the fortune of Caesar.” The great King is the one King and Lord, the Father of all (IRENAEUS).
Those who color their hair do profane the word of the Lord, “You cannot make one hair white or black” (TERTULLIAN, CYPRIAN).
Light is light; and darkness, darkness; yes is yes; and no, no (TERTULLIAN).
Let an oath be absent in everything (CLEMENT OF ALEXANDRIA).
He will everywhere guard the truth (LACTANTIUS).