I speak not by commandment, but by occasion of the forwardness of others, and to prove the sincerity of your love.
Not by commandment - `not by way of commandment.'
By occasion of х dia (G1223) with the genitive, by means of, not on account of]
The forwardness of others ... - `but by (mention of) the forwardness of others (as an inducement to you), and to prove (literally, proving) the sincerity of your love.' Bengel, etc., 'By means of the forwardness of others, proving the sincerity of your love ALSO.' The former is simpler.