“One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind.”—Rom. 14:5.
“Ye observe days, and months, and times, and years. I am afraid of you, lest I have bestowed upon you labour in vain.”—Gal. 4:10, 11.
The key to this apparent difficulty will be found in attending to the persons addressed. The Roman and Galatian churches were each composed of both Jews and Gentiles; but they are not addressed promiscuously; neither are they the same description of people who are addressed in both passages. Those who regarded days among the Romans were the converted Jews, who, having from their youth observed them as Divine appointments, were with difficulty brought to lay them aside. And as their attachment had its origin in a tender regard to Divine authority, they were considered as keeping the day unto the Lord; and great tenderness was enjoined upon the Gentile converts towards them in that matter.
Those, on the other hand, who among the Galatians “observed days, and months, and times,” were converted Gentiles, as is manifest from the context, which describes them as having, in their unconverted state, done “service to them which by nature were no gods,” ver. 8. These, being perverted by certain judaizing teachers, were, contrary to the apostolical decision, (Acts 15,) circumcised, and subjected themselves to the yoke of Jewish ceremonies. Nor was this all: they were brought to consider these things as necessary to justification and salvation, which was subversive of the doctrine of justification by faith in Jesus Christ, Acts 15:1; Gal. 5:4.
Considering these differences, the different language of the apostle is perfectly in character. Circumcision, and conformity to the laws of Moses, in Jewish converts, was held to be lawful. Even the apostle of the Gentiles himself to the Jews became a Jew, frequently, if not constantly, conforming to the Jewish laws; and writing to others he expresses himself on this wise: “Is any man called being circumcised? Let him not become uncircumcised. Is any called in uncircumcision? Let him not become circumcised. Circumcision is nothing, and uncircumcision is nothing; but keeping of the commandments of God.” But for Gentiles, who had no such things to be alleged in their favour, to go off from the liberty granted to them, (Acts 15,) and entangle themselves under a yoke of bondage—and not only so, but to make it a term of justification—was sufficient to excite a fear lest the labour which he had bestowed upon them was in vain.
Fuller, A. G. (1988). The Complete Works of Andrew Fuller: Memoirs, Sermons, Etc. (J. Belcher, Ed.) (Vol. 1, pp. 667–684). Harrisonburg, VA: Sprinkle Publications.