I want to propose a new consideration for being Reformed-- "historia salutis".
Most of us who are Reformed are so because of the categories of systematic theology: like total depravity, God's sovereignty, unconditional election. This are all good true and right reasons for being Reformed. We find these doctrines in the Word of God.
Last Easter, I was preaching through John 17. It unfolded the Trinity and the work of the Trinity at the center of redemption. What is accomplished is applied for those who for whom it is accomplished.
We have this grand picture of the Triune God working at the climax of redemption.
Somewhere along the way I released that just as much as the "order of salvation" (eternal election, effectual calling, regeneration, justification, etc.) or the "systematic topics" drive me to be "reformed"--I also have just as many reason in historia salutis for being Reformed.
You see there would be no application of redemption if there was not redemption. The better I understand Christ's death, resurrection, ascension and session at God's right hand the more I am driven to the cores of Reformed theology.
Maybe at this point I am indebted to Bavinck and his linking of these aspects. Of course, Bavinck is not alone in Reformed theology.
As I consider "why I am Reformed" it is not just the case that I am reformed because I find these doctrines in Scriptures. It is even more: these doctrines revealed in Scripture are grounded themselves on what Christ actually did in history. This I think is the emphasis of Geerhardus Vos.
The more I contemplate the work of Christ, I am Reformed not just because of the way Jesus applies redemption but what he actually does when he does those things in history. Redemptive-history itself (not just the explication of what happened but the history itself that is the source of all these wondrous effects) cause me to be more confident in Reformed soteriology.