Judges 3:2 (this was only so that the generations of the children of Israel might be taught to know war, at least those who had not formerly known it)
God left some of His enemies in the land of Israel so that He could force His people to learn war. Why? Israel went through lengthy periods of peace, but only after they won a war. The United States has never had forty continuous years of peace in its entire history, but the book of Judges describes various times of forty and even eighty years of peace in Israel AFTER they had won a war.
If you start accomplishing something in your Christian life, you're likely to have war. Jealous individuals start demanding leadership positions over successful Christian works, and Christians who are doing little or nothing for the Lord start criticizing those who are. It is inevitable that if God starts using you, you are going to have problems.
Over the years, I have seen several occasions in which a Christian would be successful at some work for the Lord, someone would start criticizing, and the Christian would quit. No bad testimony for anybody; but the work stopped. If you want to sicceed in God's work, you need to get tough. Endure the hardship, and continue with the Lord's work.