As a Christian writer, you have no shortage of inspiration available to you. Through prayer, Bible reading, worship, and enjoying God’s presence in nature or in Christian fellowship, not to mention looking at all the names of God, all the testimonies of what He has done, and all the needs lifted up to Him, you have an endless supply of topics to write about.
Sometimes the inspiration is so vast and deep, you may need help to find a simple starting point. Your first starting point should always be prayer and allowing God to lead you. Sometimes it also helps to pray over specific writing prompts.
Here is a good resource that can help you in this process: Abingdon Theological Companion to the Lectionary (published in three volumes, A, B, and C), Paul Scott Wilson, editor.