God created us to serve Him. In our love for Him, it is His will that we tell others of His saving grace. If you want to call this purpose, that's fine. Romans 8:28, "And we know that in all things God works for the good of those who love him, who have been called according to his purpose."
sorry to split hairs, but God did not create us foremost to serve Him. He created us foremost to glorify/love Him. If serving Him were the main reason, He could have made robots. and your Romans quote seemed out of left field to me. with love but this stuff's important.