Rise up, set out on your journey and go over the Valley of the Arnon. Behold, I have given into your hand Sihon the Amorite, king of Heshbon, and his land. Begin to take possession, and contend with him in battle.--Deuteronomy 2:24 (ESV)
Deuteronomy 2 tells the story of the Israelites' wanderings through the wilderness for forty years. One of the main themes of the chapter is God's sovereignty over kingdoms, conquests, and wars. When the Lord commanded the Israelites to attack the Amorites in Heshbon, He promised to give them victory. But, interestingly enough, He also commanded them to fight. The Israelites' battle was the means God used to defeat the Amorites. He could have smitten them as He did the Assyrians years later (2 Chronicles 32). But He chose to use the Israelites' faith-testing effort to accomplish His plan.
The same principle guides our sanctification. God is the Agent of sanctification in our souls. He puts new desires in our hearts (Ezekiel 11:19; Jeremiah 31:33); He protects us from temptation (Matthew 6:13); He gives us strength to resist the temptations which do afflict us (1 Corinthians 10:13)--He sanctifies us (Exodus 31:13; Leviticus 21:8; John 17:17; 1 Thessalonians 5:23).
But, this sanctification is a synergistic process. God commands us to actively pursue the sanctification which He is working in us. Look at 2 Corinthians 7:1, "let us cleanse ourselves from every defilement of body and spirit, bringing holiness to completion in the fear of God." Let us cleanse ourselves. Time and time again throughout the Bible, God commands us to sanctify ourselves--for example, to adopt a new practice or to forsake a sinful habit.
How do we make sense of this? It's a mystery of God. I think one could study Philippians 2:12-13 for years and still not comprehend how both our actions and God's are necessary for our sanctification. The good thing is that we don't need to fully understand this mystery to enjoy its rewards. We know that God will sanctify us. We can trust that He is working in our hearts right now. But we also know that He has commanded us to "work out your own salvation with fear and trembling" (Philippians 2:12). So we approach each day with (1) faith that God will change us and (2) vigilance to work hard for our sanctification.