Substitute IIF( for IF, a comma for THEN and another comma for ELSE and place a close paren ')' at the end.
IIF syntax is IIF( check expression, true expression, false expression)
You will also want to strip out the { and } around the field names and use SSRS conventions for the fields Fields!fieldname.Value
HTH
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[font="Arial Narrow"]Numbers 6:24-26[/font]