The Didache was pretty much the first catechism and was written in the mid-first century, possibly before much of the New Testament. It made its way around the early Church and was read during liturgical services (Mass).
Here are the first three chapters to whet your appetite…
Chapter 1. The Two Ways and the First Commandment. There are two ways, one of life and one of death, but a great difference between the two ways. The way of life, then, is this: First, you shall love God who made you; second, love your neighbor as yourself, and do not do to another what you would not want done to you. And of these sayings the teaching is this: Bless those who curse you, and pray for your enemies, and fast for those who persecute you. For what reward is there for loving those who love you? Do not the Gentiles do the same? But love those who hate you, and you shall not have an enemy. Abstain from fleshly and worldly lusts. If someone strikes your right cheek, turn to him the other also, and you shall be perfect. If someone impresses you for one mile, go with him two. If someone takes your cloak, give him also your coat. If someone takes from you what is yours, ask it not back, for indeed you are not able. Give to every one who asks you, and ask it not back; for the Father wills that to all should be given of our own blessings (free gifts). Happy is he who gives according to the commandment, for he is guiltless. Woe to him who receives; for if one receives who has need, he is guiltless; but he who receives not having need shall pay the penalty, why he received and for what. And coming into confinement, he shall be examined concerning the things which he has done, and he shall not escape from there until he pays back the last penny. And also concerning this, it has been said, Let your alms sweat in your hands, until you know to whom you should give.
Chapter 2. The Second Commandment: Grave Sin Forbidden. And the second commandment of the Teaching; You shall not commit murder, you shall not commit adultery, you shall not commit pederasty, you shall not commit fornication, you shall not steal, you shall not practice magic, you shall not practice witchcraft, you shall not murder a child by abortion nor kill that which is born. You shall not covet the things of your neighbor, you shall not swear, you shall not bear false witness, you shall not speak evil, you shall bear no grudge. You shall not be double-minded nor double-tongued, for to be double-tongued is a snare of death. Your speech shall not be false, nor empty, but fulfilled by deed. You shall not be covetous, nor rapacious, nor a hypocrite, nor evil disposed, nor haughty. You shall not take evil counsel against your neighbor. You shall not hate any man; but some you shall reprove, and concerning some you shall pray, and some you shall love more than your own life.
Chapter 3. Other Sins Forbidden. My child, flee from every evil thing, and from every likeness of it. Be not prone to anger, for anger leads to murder. Be neither jealous, nor quarrelsome, nor of hot temper, for out of all these murders are engendered. My child, be not a lustful one. for lust leads to fornication. Be neither a filthy talker, nor of lofty eye, for out of all these adulteries are engendered. My child, be not an observer of omens, since it leads to idolatry. Be neither an enchanter, nor an astrologer, nor a purifier, nor be willing to took at these things, for out of all these idolatry is engendered. My child, be not a liar, since a lie leads to theft. Be neither money-loving, nor vainglorious, for out of all these thefts are engendered. My child, be not a murmurer, since it leads the way to blasphemy. Be neither self-willed nor evil-minded, for out of all these blasphemies are engendered.
Rather, be meek, since the meek shall inherit the earth. Be long-suffering and pitiful and guileless and gentle and good and always trembling at the words which you have heard. You shall not exalt yourself, nor give over-confidence to your soul. Your soul shall not be joined with lofty ones, but with just and lowly ones shall it have its intercourse. Accept whatever happens to you as good, knowing that apart from God nothing comes to pass.