I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship. - Romans 12.1
Therefore and by the mercies of God both point to the previous exposition of God's compassion in action as the ground for the exhortation. The grace of God, expounded in chaps. 1-11, expect a certain response. Apart from this grace this response would not only be unanticipated, but also impossible. Thus, the mercies of God are not merely a motive for the obedience enjoined in chaps. 12-15, but the fountain from which it flows or the soil in which it grows. It is instructive that Paul does not refer to the economy of salvation just expounded as "doctrines" but as "mercies" or more fully "compassion in action."
The words present . . . . sacrifice and worship are cultic concepts derived from the OT worship at the Tabernacle/Temple. The product of and proper response to God's grace in Christ is worship. That this is not limited to activities at particular sacred spaces and sacred times is made clear by the object (your bodies) and the character (spiritual) of the worship.
The use of the word bodies makes the imagery of OT sacrifice all the sharper, and emphasizes the totality of devotion called for. All of our existence, in everything we bump into in this life, is to be offered to God. The word spiritual or reasonable clarifies that this cannot be merely external conformity, or going through the motions. The use of these two words (bodies and spiritual) together can correct perverse thoughts about the Christian life. On the one hand, there are those who believe that what they do matters very little as long as they "believe." Sinful behavior is often excused by saying, "God knows my heart." But the word of God says, "present your bodies" and "glorify God in your body" (1 Cor 6.20). On the other hand are those who pride themselves on their external conformity to the commands of Scripture, but are not inwardly changed. True worship is spiritual or reasonable, as Douglas Moo explains, "in the sense of 'appropriate for human beings as rational and spiritual creatures of God': a worship that honors God by giving him what he truly wants as opposed to the depraved worship offered by human beings under the power of sin (see Rom 1.23-25)." (Moo, Romans, NICNT).
This sacrifice of our bodies is living because we have been raised with Christ to walk in newness of life (6.4-11). This sacrifice is holy because, having been set free from sin, we are set apart as God's servants (6.22). This sacrifice is acceptable because we are not in the flesh but in the Spirit, "if in fact the Spirit of God dwells in you" (8.8-9).
Do not be mislead by those expositors who wrongly say that the aoristic aspect of present implies a once-for-all presentation of the self to God. The aorist is the default tense of the infinitve in the NT, hense it is the least marked tense. If the present or perfect tense were used, then something could be said regarding the kind of action. The aorist alone does not define the kind of action (progressive, perfected, etc.). The fact that Paul is writing to a church whose members would no doubt be at different maturity levels spiritually is a good argument for seeing this presentation as a repeatable discipline, not a one-time act.
In light of the dawning of a new year and this exhortation of Holy Spirit, let us resolve, as Jonathan Edwards did on the twelfth day of January, 1723, "frequently to renew the dedication of myself to God."