"I write to you to love one other, which is not a new thing to do but an old commandment that you had heard." John refers to Jesus' command to love one another.
Here, "beginning" refers to when they decided to follow Christ. AT: "from when you first believed in Christ" (See: [[rc://en/ta/man/translate/figs-explicit]])
"Word" here is a synecdoche for "message." AT: "The old commandment is the message that you heard. (See: [[rc://en/ta/man/translate/figs-synecdoche]]).
Here "darkness" refers to evil and "light" refers to goodness. AT: "because you are ceasing to do evil and you are doing more and more good" (See: [[rc://en/ta/man/translate/figs-metaphor]])