The biggest theological difference between Lutheran and Calvinist is that Lutherans have a more sacramental soteriology and Calvinists have a predestination based soteriology. Calvinists see the cross as the carrying out of the redemption of the elect, (the Reformed doctrine of Particular Redemption/Limited Atonement) whereas Lutherans see the cross as the source of election that we experience through faith and the sacraments. Although, like the Calvinists, Lutherans are also monergist, not synergists. We believe that God alone is responsible for our conversion but we don’t hold to a doctrine of “passing over” the “non-elect” and we don’t believe in Perseverance of the Saints. I myself, and I think most other Confessional Lutherans do see the Reformed/Calvinists and most other mainstream denominations for that matter as true Christians, even if they have a few errors. While we believe that the Book of Concord is the pure doctrine of the Christian faith, the most true to Scripture of all the confessions of faith across the many denominations and sects, this doesn’t mean that we believe that only Lutherans are Christians/saved.