“I will begin to make you great.” God and the Devil both empower people so that they are great in the eyes of others, but being elevated by the Devil brings only short success and then a very heavy price, while being elevated by God brings blessings now and in the future. That the text states that it is God who makes Joshua great is part of Joshua’s being a type of Christ, because Christ did the works of God and was magnified by God (cp. Acts 10:38).
“so that they will know.” A part of the reason that God elevated Joshua in the eyes of Israel the way He did was so that Israel would know that God was with Joshua. Similarly, the works that Jesus did testified that he was sent by God (John 5:36; 10:25; 14:10-11). When God is obviously working in someone’s life but scoffers deny and denigrate it, that is sin (John 15:22-24).