Adam Clarke on Romans 14:17

For the kingdom of God - That holy religion which God has sent from heaven, and which be intends to make the instrument of establishing a counterpart of the kingdom of glory among men: see on Mat 3:2 (note).

Is not meat and drink - It consists not in these outward and indifferent things. It neither particularly enjoins nor particularly forbids such.

But righteousness - Pardon of sin, and holiness of heart and life.

And peace - In the soul, from a sense of God's mercy; peace regulating, ruling, and harmonizing the heart.

And joy in the Holy Ghost - Solid spiritual happiness; a joy which springs from a clear sense of God's mercy; the love of God being shed abroad in the heart by the Holy Ghost. In a word, it is happiness brought into the soul by the Holy Spirit, and maintained there by the same influence. This is a genuine counterpart of heaven; righteousness without sin, Peace without inward disturbance, Joy without any kind of mental agony or distressing fear. See the note on Mat 3:2.