Therefore you are no longer a slave but a son, and if a son, then an heir of God through Christ.
—Galatians 4:7

It is surprising that some people call the gospel of grace, which the apostle Paul preached and in which the righteousness of God is revealed [Rom.1:16-17], the elementary teaching of Christ for so-called “baby Christians”, referring to John’s epistle (although it isn’t about a Christian’s age at all), while they regard the teachings of Jesus in the Gospels as food for grown-ups. John did not divide believers into three categories by any criteria. Think about it: if the things listed in 1 John 2:12-14 are criteria of maturity, then there is no difference between the little children and the fathers, because the little children have known the Father, and the fathers Him who is from the beginning [Jesus]. As if knowing the Father were somehow different from knowing Jesus. [Jhn.14:8-9] We mustn’t forget that John was writing first of all to Jews, and among the Jews this was the customary way of addressing people. [Act.7:2; Act.22:1]

As for the Lord’s teachings in the Gospels, Jesus taught the law to Jews who were under the law and not born again, because the law was still in force, the Lord had not yet been crucified, and the Holy Spirit had not yet been given. If anyone doubts this, the essence of that same Sermon on the Mount is “Love your neighbour as yourself.” And the commandments of Moses “are all summed up in this saying, namely, ‘You shall love your neighbor as yourself.’” [Rom.13:9] The point of most of Jesus’ sermons was like what He said to the young ruler: “if you want to enter into life, keep the commandments… If you want to be perfect, go, sell what you have” [Mat.19:16-21], or as in the Sermon on the Mount: “that you may be sons of your Father in heaven.” [Mat.5:45]

Living under the law, or under the commandment “Love your neighbour as yourself”, or under the [law] teachings of Jesus from the Gospels is, in the words of our precious Paul, “childhood”, which does not differ at all from slavery. [Gal.4:1-5] Grown-up sons are those who have received the Holy Spirit, the Spirit of adoption, by whom we cry out, “Abba, Father!” And to be closer to the text: we received the Spirit because we became grown-ups. [Gal.4:6-7] It is by Him that we are led, and “if you are led by the Spirit, you are not under the law.” [Gal.5:18] So if you preach the gospel of grace and reveal the righteousness of Jesus, and people answer that all this is for “baby Christians”, know that you are no longer children [slaves] but grown-ups [sons], and therefore heirs of God through Jesus Christ.