Matt 3:15 says:
“Let it be so now,” Jesus replied. “It is fitting for us to fulfill
all righteousness in this way.” Then John permitted Him.
Ignatius almost quotes this passage when he says in the letter to the Smyrneans, 1:1,
… The Son of God was … baptized by John in order that all
righteousness might be fulfilled by him
BDAG suggests that word translated "righteousness" here, δικαιοσύνη (dikaiosuné), means (#3b) the quality or characteristic of upright behaviour, uprightness, righteousness; … of specific action, righteousness in the sense of fulfilling divine expectation not specifically expressed in ordinances, eg, Matt 3:15, 5:20, 6:1, 2 Cor 9:9 (the concern and care of the poor).
I think this is essentially saying that here is one place that righteousness must exceed that of the written code (Matt 5:20) of the Torah - to do all things, even those not written down! The commentaries also reach a similar conclusion:
All righteousness - There was no particular precept in the Old
Testament requiring this, but he chose to give the sanction of his
example to the baptism of John, as to a divine ordinance. The phrase
"all righteousness," here, is the same as a righteous institution or
appointment. Jesus had no sin. But he was about to enter on his great
work. It was proper that he should be set apart by his forerunner, and
show his connection with him, and give his approbation to what John
had done. He submitted to the ordinance of baptism, also, in order
that occasion might be taken, at the commencement of his work, for God
publicly to declare his approbation of him, and his solemn appointment
to the office of the Messiah.
All righteousness (πᾶσαν δικαιοσύνην). Not the whole circle of
righteousness (πᾶσαν τὴν δικαιοσύνην), but every part of righteous
ness, as each is presented to us (similarly, Acts 13:10; cf. also
δικαιοσύναι in Ecclus. 44:10; Tobit 2:14, where, although Neubauer and
Fuller explain it as "alms." this is improbable after the preceding
ἐλεημοσύναι), and that not merely every part of the righteousness
included under the Mosaic, Law (cf. Alford, "requirements of the Law'
and especially Lowe. 'Pesach Fragm.,' p. 100: 1879), but of that wider
righteousness of which that was itself only a part and a type. "Let me
be baptized by thee now," our Lord says to John, "for it is fitting
for us, in this spirit of submission, to fill up every part of
Thus, Jesus is pointing to a greater righteousness that transcends and exceeds the written law of the Torah - He is describing a righteousness not from without but a righteousness that God creates within us that is planted and becomes innate by a miracle of the Holy Spirit - a change of heart and attitude. It is described in various ways such as "the mind of Christ", the law of love", "the law of Christ", etc.