The Greeks had three words that we translate, love--agape, phileo, and eros. Phileo refers more to a "friendship" love, eros to an erotic love, and agape to a sacrificial, unconditional love.
When we talk of loving God, often we mistakenly think of our love for God in terms of phileo love (or worse yet, eros love). We have nice feelings for God. We like Him. But, agape love is the love with which God loves us, and it is the love with which we have been called to love God and others.
What does it look like to love God with agape love? First John 4:7–12 (NET) says:
Beloved, let us love one another, for love is from God, and whoever loves has been born of God and knows God. 8 Anyone who does not love does not know God, because God is love. 9 In this the love of God was made manifest among us, that God sent his only Son into the world, so that we might live through him. 10 In this is love, not that we have loved God but that he loved us and sent his Son to be the propitiation for our sins. 11 Beloved, if God so loved us, we also ought to love one another. 12 No one has ever seen God; if we love one another, God abides in us and his love is perfected in us.
Wolfhart Pannenberg asks some provocative questions about Christian love for God:
If Christian love is essentially a participation in God's love for the world, then we have to ask whether we can distinguish at all between love of God and love of neighbor. Does not true love consist of sharing in God's love for the world? And in the depth of turning to the cohuman Thou, do we not also love God? (Systematic Theology, vol. 3, 187.)
Pannenberg argues that, while love for God and love for neighbor are inseparable, they do not collapse into one another. Jesus seemed to prioritize love for God over love for others, implying at least a subtle distinction. Pannenberg describes love for God:
We love God by letting him be God to us as Jesus let the Father be God to him, by letting him be our God, our Father, and thus by putting our trust and confidence in him. (Systematic Theology, vol. 3, 193.)
Love for God is expressed in faith.