Deuteronomy 6:5 (also Mt 22:37, Mark 12:30, Luke 10:27)
Love the LORD your God with all your heart and with all your soul and with all your strength.
1 Peter 2:17 (also Deut 6:13, Ps 33:8, 34:9, 111:10)
Show proper respect to everyone: Love the brotherhood of believers, fear God, honor the king.
I tell you, my friends, do not be afraid of those who kill the body and after that can do no more. But I will show you whom you should fear: Fear him who, after the killing of the body, has power to throw you into hell. Yes, I tell you, fear him.
1 John 4:16-18
God is love. Whoever lives in love lives in God, and God in him. In this way, love is made complete among us so that we will have confidence on the day of judgment, because in this world we are like him. There is no fear in love. But perfect love drives out fear, because fear has to do with punishment. The one who fears is not made perfect in love.
The apparent contradiction here is that we are told to both love and fear God, and yet 1 John says there is no fear in love. However, the word "fear" can mean different things. According to Dictionary.com, one definition of "fear" is "reverential awe, especially toward God." Reverential awe, or profound respect, is compatible with love.
This particular meaning of "fear" is used in Psalm 130:4, which says, "But with you there is forgiveness, that you may be feared." One doesn't need to be terrified of someone who offers forgiveness. But since God is the only one who has the authority to forgive sins, we are dependent on his forgiveness and this should inspire us to revere him. 1 Peter 2:17 and the verses listed next to it also use this meaning of "fear", as is evident from their context. For example, Psalm 33:8 says, "Let all the earth fear the LORD; let all the people of the world revere him."
1 John 4:18 uses "fear" in the usual sense, as is also evident from context - punishment is something to be afraid of. Thus Christians are to love God and stand in awe of him, but not to be terrified of him.
A stronger case for contradiction can be made with Luke 12:5. Like 1 John 4:18, it's talking about fear of punishment, and thus is using "fear" in the same sense. However, looking at the message of both passages resolves the contradiction. In Luke, Jesus is telling "a crowd of many thousands" (Lk 12:1), which would have included both believers and non-believers, to fear God who has the ability to punish them in hell. John, apparently writing to believers, reminds them that because of God's love, "we will have confidence on the day of judgment." In other words, "perfect love drives out fear, because fear has to do with punishment." Those who love God and believe in him know that he loves them and can trust in him to save them. Thus unbelievers have reason to be afraid of God; believers are not to be afraid because of their trust in God's love (fear on the part of believers is a sign of doubt or inadequate knowledge of God - "the one who fears is not made perfect in love.").