Rom 14:1-4
"As for the one who is weak in faith, welcome him, but not to quarrel over opinions... One who eats is not to despise the one who abstains, and one who abstains is not to pass judgment on the one who eats, for God has welcomed him."
The context: probably Jewish Christians with scruples about food sacrificed to idols. Paul does not take a side on the issue itself — he addresses the attitude: neither the 'strong' should despise the 'weak,' nor the 'weak' should judge the 'strong.'
Rom 14:17-19
"For the kingdom of God is not a matter of eating and drinking but of righteousness and peace and joy in the Holy Spirit."
The kingdom of God is not defined by external practices — it is defined by righteousness, peace, and joy in the Holy Spirit. Christian freedom must be exercised for the sake of the edification (oikodome) of the other.