While it is true that Abraham believed God, so it was counted to him as righteousness (Genesis 15:6), it is also true that he believed God, so he obeyed God's command to offer Isaac (Hebrews 11:17), so the same faith by which he was declared righteous was also expressed as being an obeyer of God, but he did not earn his righteousness as a wage as the result of his obedience (Romans 4). In James 2:21-24, he quoted Genesis 15:6 to support saying that Abraham was justified by his works when he offered Isaac, his faith was active along with his works, and his faith completed his works, so he was justified by his works insofar as they were expressing his faith, but not insofar as they were earning it as a wage.
In Acts 21:20, they were rejoicing that tens of thousands of Jews were coming to faith who were all zealous for the Mosaic Law, which is in accordance with believing the Gospel that Christ taught in Matthew 4:15-23 to repent for the Kingdom of God is at hand and with believing in what Christ accomplished in Titus 2:14 in that he gave himself to redeem us from all lawlessness and to purify for himself a people of his own possession who are zealous for doing good works.