When we talk about what is good, in the long run what we say has to do with God. Writing about Mt 19:16-21, Pope John Paul II says:
Jesus says: “Why do you ask me about what is good? There is only one who is good. If you wish to enter into life, keep the commandments” (Mt 19:17). In the versions of the Evangelists Mark and Luke the question is phrased in this way: “Why do you call me good? No one is good but God alone” (Mk 10:18; cf. Lk 18:19).
Before answering the question, Jesus wishes the young man to have a clear idea of why he asked his question. The “Good Teacher” points out to him — and to all of us — that the answer to the question, “What good must I do to have eternal life?” can only be found by turning one’s mind and heart to the “One” who is good: “No one is good but God alone” (Mk 10:18; cf. Lk 18:19). Only God can answer the question about what is good, because he is the Good itself.
To ask about the good, in fact, ultimately means to turn towards God, the fullness of goodness. Jesus shows that the young man’s question is really a religious question, and that the goodness that attracts and at the same time obliges man has its source in God, and indeed is God himself. God alone is worthy of being loved “with all one’s heart, and with all one’s soul, and with all one’s mind” (Mt 22:37). He is the source of man’s happiness. Jesus brings the question about morally good action back to its religious foundations, to the acknowledgment of God, who alone is goodness, fullness of life, the final end of human activity, and perfect happiness. [Veritatis Splendor §9; emphasis in original]
When we talk about what is good, we can’t get away from the fact that goodness ultimately is found in God. Any conversation about other goods must be understood relatively, and to the extent that they are contrary to God’s goodness they aren’t real goods at all. If we absolutize anything else as the final good, then we make that thing into an idol. So if we attempt to establish a morality upon any other principle than God Himself, we are propping up something that is false in its foundations.
That doesn’t meant that individual aspects of such a morality might not be good. One does not need to be a Christian in order to know that murder is wrong, for example (although he might err as to the fundamental reasons why this is so). But for Christians no other form of morality (i.e., any not founded upon God) can be held to be legitimate, because they amount to idolatry.
Likewise it is no part of genuine morality to speak of something as good that is contrary to God’s goodness. More will be said about this later in the encyclical, but for example deliberate abortion can never be said to be a morally acceptable option even in a relative sense.