John 16:33

“These things I have spoken to you so that in me you may have peace. In the world you have tribulation. But be of good cheer, I have overcome the world.”

His purpose in all that He has said is so that in the end they will have peace. As they think back and remember all He said and all that has happened their assurance and confidence will grow and peace will fill their hearts. And especially He wants them to have confidence in the fact that the world will not win. It is God Who will win. For by His sacrifice of Himself He has overcome the world and all that it stands for. The point would appear to be that His light will triumph over the world's darkness and over its evil intent (John 1:4).

‘In the world you have tribulation.' That will be true for them and for the church and Christians throughout the centuries. ‘Tribulation' is the Christian's lot because he is at enmity with the world's ways. The word means ‘distress brought by outward pressure'. But its purpose is good for it produces patient endurance, leading to experience which results in hope for the future (Romans 5:3).