CCC 129 Christians therefore read the Old Testament in the light of Christ crucified and risen. Such typological reading discloses the inexhaustible content of the Old Testament; but it must not make us forget that the Old Testament retains its own intrinsic value as Revelation reaffirmed by our Lord himself.1 Besides, the New Testament has to be read in the light of the Old. Early Christian catechesis made constant use of the Old Testament.2 As an old saying put it, the New Testament lies hidden in the Old and the Old Testament is unveiled in the New.3

CCC 202 Jesus himself affirms that God is “the one Lord” whom you must love “with all your heart, and with all your soul, and with all your mind, and with all your strength”.4 At the same time Jesus gives us to understand that he himself is “the Lord”.5 To confess that Jesus is Lord is distinctive of Christian faith. This is not contrary to belief in the One God. Nor does believing in the Holy Spirit as “Lord and giver of life” introduce any division into the One God:
We firmly believe and confess without reservation that there is only one true God, eternal infinite (immensus) and unchangeable, incomprehensible, almighty and ineffable, the Father and the Son and the Holy Spirit; three persons indeed, but one essence, substance or nature entirely simple.6

CCC 575 Many of Jesus’ deeds and words constituted a “sign of contradiction”,7 but more so for the religious authorities in Jerusalem, whom the Gospel according to John often calls simply “the Jews”,8 than for the ordinary People of God.9 To be sure, Christ’s relations with the Pharisees were not exclusively polemical. Some Pharisees warn him of the danger he was courting;10 Jesus praises some of them, like the scribe of Mark 12:34, and dines several times at their homes.11 Jesus endorses some of the teachings imparted by this religious elite of God’s people: the resurrection of the dead,12 certain forms of piety (almsgiving, fasting and prayer),13 the custom of addressing God as Father, and the centrality of the commandment to love God and neighbor.14

CCC 2196 In response to the question about the first of the commandments, Jesus says: “The first is, ‘Hear, O Israel: The Lord our God, the Lord is one; and you shall love the Lord your God with all your heart, and with all your soul, and with all your mind, and with all your strength.’ The second is this, ‘You shall love your neighbor as yourself.’ There is no other commandment greater than these.”15
The apostle St. Paul reminds us of this: “He who loves his neighbor has fulfilled the law. The commandments, ‘You shall not commit adultery, You shall not kill, You shall not steal, You shall not covet,‘ and any other commandment, are summed up in this sentence, ‘You shall love your neighbor as yourself.’ Love does no wrong to a neighbor; therefore love is the fulfilling of the law.”16

1 Cf. Mk 12:29-31
2 Cf. I Cor 5:6-8; 10:1-11.
3 Cf. St. Augustine, Quaest. in Hept. 2, 73: PL 34,623; Cf. DU 16.
4 Mk 12:29-30
5 Cf. Mk 12:35-37.
6 Lateran Council IV: DS 800.
7 Lk 2:34.
8 Cf. Jn 1:19; 2:18; 5:10; 7:13; 9:22; 18:12; 19:38; 20:19.
9 Jn 7:48-49.
10 Cf Lk 13:31.
11 Cf. Lk 7:36; 14:1.
12 Cf. Mt 22:23-34; Lk 20:39.
13 Cf. Mt 6:18.
14 Cf. Mk 12:28-34.
15 Mk 12:29-31; cf. Deut 6:4-5; Lev 19:18; Mt 22:34-40; Lk 10:25-28.
16 Rom 13:8-10.