What does Mark 1:11 mean?

The voice from heaven is clearly God the Father, and this is not the only time that God calls down from heaven to declare that Jesus is His beloved (agapetos) Son. This will occur again at the Transfiguration, when Jesus is glorified and Peter, James, and John begin to see Him as He really is (Matthew 17:5).

The Bible recounts God's voice coming from the heavens in several other events. In Genesis 21:17, He comforts Hagar after Sarah banishes her. In John 12:28, God reaffirms that Jesus' sacrifice will glorify Him. On Sinai, God spent forty days talking to Moses and giving him the law by which the Israelites were to live (Nehemiah 9:13). Because we have the completed Scriptures, God does not primarily speak to us audibly, but He does use the Bible, circumstances, and the words of other Christians to comfort, encourage, and train us.

Islam, in particular, has a difficult time accepting this verse. Islam teaches strict monotheism, and the concept of God having a "Son" worthy of worship sounds like polytheism. In later passages, Jesus will go on to teach that "The Lord our God, the Lord is one" (Mark 12:29) and that He "and the Father are one" (John 10:30). The concept of the Trinity—three persons but one God—is not easy to understand, but it is clearly taught in the Bible.