A.T. Robertson Commentary Romans 14:6

"He that regardeth the day, regardeth it unto the Lord: and he that eateth, eateth unto the Lord, for he giveth God thanks; and he that eateth not, unto the Lord he eateth not, and giveth God thanks." — Romans 14:6 (ASV)

Regardeth (φρονε). "Thinks of," "esteems," "observes," "puts his mind on" (from φρην, mind). The Textus Receptus has also "he that regardeth not," but it is not genuine.

Unto the Lord (κυριω). Dative case. So as to τω θεω (unto God). He eats unto the Lord, he eats not unto the Lord. Paul's principle of freedom in non-essentials is most important. The Jewish Christians still observed the Seventh day (the Sabbath). The Gentile Christians were observing the first day of the week in honour of Christ's Resurrection on that day. Paul pleads for liberty.