There are three great truths concerning thanksgiving taught in this little verse. Thanksgiving is not to be limited to special occasions or to a special day. We are to be giving thanks in everything. Just as we are to rejoice evermore and to pray without ceasing, so we are to be giving thanks always. Thanksgiving in everything also means thanksgiving for everything.
Nothing lies outside the scope of our thanksgiving.
And precisely for this reason thanksgiving is possible only for believers. Giving thanks is a profoundly spiritual activity in faith. And, faith is God's gift. This does not mean we give thanks for some things but not for others. We have a bad practice in this respect.