To trust in the LORD with all your heart means to renounce all self-reliance and place your total, unreserved confidence in God's wisdom and providence. It is an invitation to stop looking to your own understanding or earthly idols and instead rest completely in the goodness and guidance of the One who holds your life in His hands.
The instruction in Proverbs 3:5 to 'lean not upon thine own understanding' is a call to humility. Because we are, as the Reformed tradition teaches, fundamentally flawed and prone to folly, we must stop treating our own insights as an infallible guide. As noted in the Large Catechism, to have a god is to trust in something with the whole heart; if we trust in our own wisdom, we have made an idol of ourselves. We are invited to let go of the exhausting burden of trying to be our own sovereign.
When we 'acknowledge Him' in all our ways, we are living out the truth that we are loved in Christ and held by His providence. This is not a legalistic demand to be perfect, but a promise: 'he will direct thy paths' (Proverbs 3:6). Because Christ has finished the work of our salvation, we are free to trust that even in our wanderings, He is our Father. As the Heidelberg Catechism Q.28 teaches, knowing that God upholds all things allows us to be patient in adversity and thankful in prosperity, placing our firm trust in Him who will never let us go.