READ ROMANS 4-6
being fully persuaded that God had power to do what he had promised. (Romans 4:21 NIV)
This verse is talking about Abraham believing God that he'd be a father of many nations. Abraham was fully persuaded that God had power to do what He had promised.
Abraham had seen God's faithfulness manifested repeatedly in his life. He trusted that God was who He said He was and that He would do what He said He would do.
When you look back on your life, how have you seen God's hand move on your behalf? Knowing that He is faithful and that He never changes even though your circumstances may change, you can trust Him to care for you. You too can be fully persuaded that God has the power to do what He says He will do.
What holds you back from being fully persuaded to trust God--His power, His timing, His love, His goodness?