Are Brethren marcianists?

Are the Brethren marcianists in their total concentration on the gospel and NT only.?

I go to a gospel hall as there's no other church in our village. They are Open Brethren. Not cultish at all. Not Plymouth Brethren.

I find the people lovely, but feel they are in the wrong road because they know nothing if the OT teachings.they concentrate on the New Testement and it's very simplified sort of "Jesus loves you" Christianity.

Whilst there's nothing wrong with that in its self, I feel like they are missing a lot of who Christians are.

What do you think? Do I need to move on?