Have you ever played soccer? One person on each team is designated as the goalie, and it is the responsibility of the goalie to deflect shots at the goal. Even if you haven’t played soccer, have you ever encountered hostile or toxic words or thoughts? You could think of this the same way; that you need to deflect these things lest they be trapped in your net. David expressed the same concept as being a doorkeeper. The doorkeeper regulates admission; letting in those who are welcome and keeping out those who are undesirable. You need to perform the same function in your life, whether you think of it as a goalie or a doorkeeper.
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