Thomas Boston: Union With Christ and Communion

I came across an interesting argument for union with Christ in the writings of Thomas Boston. In the course of his two-volume work An Illustration of the Doctrines of the Christian Religion, Boston makes many compelling arguments against the Lutheran understanding of justification with respect to the discussion of the doctrine of union with Christ. One of the more novel and fascinating arguments that Boston makes has to do with the Calvinistic understanding of the Lord's Supper.

If this union be not a true and real one, but a mere relative one, the sacrament of the supper is but a bare sign, and not a seal, exhibiting and applying Christ to believers. For without this real union, the feeding on Christ's body and blood truly and really in the sacrament cannot be; which yet is the doctrine of the scriptures, and of our Larger Catechisms, proved from the words of institution, 'Take, eat, this is my body.' For if there be a true and real feeding, there must be a true and real union, as there is betwixt the food and our bodies into which it is incorporated.

The only way in which we truly take the Lord's body and blood is spiritually. As Boston points out, communion involves taking Jesus into us and being spiritually united to him. If we are not truly united to Christ by faith, then, Boston is arguing, we cannot be said to spiritually partake of Jesus in communion, either, in any true sense.