13 Now may the God of hope fill you with all joy and peace in believing, that you may abound in hope by the power of the Holy Spirit.
“Believing” is how we bring joy and peace to ourselves. The experience of being filled with all joy and peace comes by believing in the God who gives hope. The Christian life is one of depending on God and His promises. When we take God at His word, we super-abound in hope (next phrase). Faith simply believes God. The more we believe, the greater our hope.
Christians are to take God at His word.
Most people believe their doubts and doubt their beliefs. Rather, we should believe our beliefs and doubt our doubts. We do this by taking God at His Word, by believing His promises.
Faith is simply an issue of believing God when He shares with us who He is and what He did for us. There is a difference in maturity between believers who trust God regularly and those who do not.