Is sanctification monergistic or synergistic (cooperative)?

I believe in a monergistic view of justification (the forgiveness of sins) — that is, Christ is solely responsible for giving us faith to believe in Christ, and we contribute nothing to our spiritual regeneration.

I believe that by my own reason or strength I cannot believe in Jesus Christ, my Lord, or come to him.  But the Holy Spirit has called me through the Gospel, enlightened me with his gifts, and sanctified and preserved me in true faith“. (Martin Luther, Small Catechism)

Be a sinner and sin boldly, but believe even more boldly and rejoice in Christ, who is victor over sin, death, and the world”. (Luther)

For me personally, the issue of justification is not an issue.  The issue is sanctification.  My question is whether sanctification is merely a question of semantics, or is there a big difference?  [ first a definition: ] Sanctification is the being made holy.  It denotes the inward spiritual transformation of the believer, whereby, one is made holy.  Justification is the source of sanctification.  Sanctification comes as a result when one becomes justified in Christ Jesus.

Evangelicals seem to focus on sanctification but it is also what trips us up.  Sanctification can also be somewhat of a tricky issue to define—is it monergistic or synergistic?  I’ve been searching but this is some of what I’ve found (Puritan Board; Monergism). Some see sanctification as a something we do in cooperation with God.  Some see it as something that God alone does for us. If we see it from a horizontal basis, then in this sense, you could say it’s synergistic, or cooperative; but on a vertical basis, it is monergistic.  It can be seen as being both; and this is why this can be a little confusing.  What is it–monergistic, synergistic, or simply cooperative?