Vatican City, May 7 - Pope Francis said Tuesday that good Christians do not grumble when life throws up problems. "When difficulties arrive, many temptations come with them. Complaining, for example," Francis said in his homily at a mass at the Saint Martha's House residence where he is staying inside the Vatican. "A Christian who continually complains, neglects being a good Christian... The silence of bearing the Cross is not a sad silence. It is painful, often very painful, but it is not sad". The Argentine pontiff also used his Twitter account on Tuesday to call on Catholics to be active followers of their faith. "Do not be content to live a mediocre Christian life: walk with determination along the path of holiness," he said via the @Pontifex account.