Christians believe in a sovereign God who ordains all things. Reformed Christians believe that salvation and growth are works of the Holy Spirit alone. It is their understanding that, as John states, nobody can come to Jesus unless the Father draws him (cf. John 6:44). Likewise, spiritual growth is Spirit-wrought process in the heart of the believer, what is termed sanctification, the process of becoming more like Jesus Christ (cf. 1 Thess 5:23; 2 Thess 2:13; Ti 3:5).
But if salvation is wrought by God alone and sanctification is a work of the Spirit in the hearts of believers, why did Christ command his followers to go into all the world and make disciples of all nations (cf. Mat 28:19)? Surely a sovereign God could save people far quicker and more easily without calling fallen, sinful people like us to join him in that work? A sovereign God could surely cause people to grow without involving us? Why does God command us to be involved in that work of making and growing disciples when we are likely to make much harder work of it, if not mess it up altogether?
Clearly, it is not for his benefit. He could, indeed, do these things without reference to us – without the fuss, mess and time involved. If it’s not for his benefit, it seems he must include us for our benefit.
It is a little like when my son asks to help my wife bake a cake. My wife is perfectly capable of baking by herself and doesn’t really need any help, least of all from our 3 year old. In fact, the cake would be made much quicker on her own and the kitchen would be less messy if my son doesn’t help. Yet, when he helps, he learns and grows. And there is no denying he does actually help, he is involved in the work of mixing, measuring and whisking. It might take longer, it might make more mess, but he benefits from being included and so my wife is glad to include him because he enjoys it whilst learning and growing in the process.
Just like that, though God could do these things quicker by himself, he calls us into the work because through it we learn, grow and even find joy. It is through serving Christ and joining in his mission that we are made like him. It is in our growth and obedience to him that we find our joy in him. God calls us into this working of making and growing disciples for our good. He call us to it for our joy.