Romans 6:3

Were baptized into Christ

(εβαπτισθημεν εις Χριστον). First aorist passive indicative of βαπτιζω. Better, "were baptized unto Christ or in Christ." The translation "into" makes Paul say that the union with Christ was brought to pass by means of baptism, which is not his idea, for Paul was not a sacramentarian. Εις is at bottom the same word as εν. Baptism is the public proclamation of one's inward spiritual relation to Christ attained before the baptism. See on Galatians 3:27 where it is like putting on an outward garment or uniform.Into his death

(εις τον θανατον αυτου). So here "unto his death," "in relation to his death," which relation Paul proceeds to explain by the symbolism of the ordinance.