Two simply words, "Thank you."  They are often used off-handed, with no real emotion or importance behind them; But when they come from the heart they're powerful.  Thankfulness and gratitude swell up from within us when we have been overwhelmed by something or someone who has done something for us.

Jesus took the punishment for my sin.  He carried my anxiety and worry and fear.  He exchanged my wickedness for His righteousness. He has given me a place with Him in heaven.  Really, a "thank you" isn't quite enough - when we realize all this our simple "thank you" become highest Praise!