For he who has entered His rest has himself also ceased from his works as God did from His."
Hebrews 4:9-10 (NKJV)
When you are traveling to a place the time will come when you that know you have arrived, the journey would be over because you had reached your destination. If you were looking for something you would come to the point when you know that you have what you were looking for, you are no longer seeking that thing but you have found it. It would be very strange to not know that you had reached your destination or that you really had what you were seeking. In both situations, there is something you have and know that indicates that your journey or search has ended.
The Bible shows us that faith is what we need to receive what God has provided. Romans 10:17 says that faith comes by hearing, and hearing by the Word of God. This shows us that we get faith from the Word of God. We also know that according to 2 Corinthians 5:7 that we walk by faith and not sight, so faith is not based on physical evidence. But, there is a way to know if we have really put our faith in God and placed our trust and confidence in His Word rather than in something else. The thing that shows us whether or not we are really in faith is rest.
One of the key elements of the Christian life is peace. This is also a good way for us to see whether or not we are really in faith. If someone is at rest then they are at peace, if there is no peace then there is no rest. If we are really trusting in what God has told us and we in faith for what the Bible has said then we are at rest, we are at peace about the situation. When we are at rest it means our confidence is in God and we are not trusting in our own works, we have put our trust in the work of Jesus. We rest in faith expecting to receive what God's Word has said; if we are in faith then we are at rest.