The beginnings of Rom. 12 and Eph. 5 are evidently different, and the difference is not only interesting, but according to the tenor of the epistles.
"I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service. And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect will of God." Rom. 12:1, 21I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service. 2And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God. (Romans 12:1‑2).
In Romans the Spirit looks for an entire separation to God as men on earth, the giving up of self in consecration to Him as a living sacrifice. I am a man here, and am to be as one offered up to God. But love, as going out, does not come in here. We living men give ourselves by grace up to God.
In Ephesians we are sitting in heavenly places, and come out from God as Christ did, and that in love. And this is the direct connection, we forgiving and forbearing, as God in Christ did. Do you therefore, says the Apostle, be imitators of God, and walk in love, as Christ did, giving Himself for us.
Hence in Romans we are responsible men, giving ourselves up to God as in the world. In Ephesians we are children of God, dear children, who are to imitate Him according to the great pattern, Christ, the Son of God, come out from the Father, giving Himself for us, which is love, only to God as a sacrifice, which maintains absolute perfection as the One in view in doing so. The new creation had been brought out before, after God, in righteousness and true holiness. Here, when the Spirit of God is brought in, love, according to the divine nature, is to display itself, and does so in a remarkable character and place. Christ being the absolutely perfect model of it. Romans put us on the ground of proving His will, Ephesians of displaying His character, going forth in love as Christ, though always with a view of pleasing God, which makes the motive perfect.