The pontiff made the remark on the second leg of his Africa tour.
Pope Francis visited Uganda President Yoweri Museveni (pictured above) at the state house in the capital Kampala.
He then celebrated Mass with more than 100,000 faithful at an open shrine in Namugongo honouring 45 Ugandan Christians killed for their faith by a tribal king between 1885 and 1887.
In light of their deaths, the Pope said all Christians should be missionaries "to our families and friends certainly, but also to those whom we do not know, especially those who might be unfriendly, even hostile, to us.
"We need only to open our eyes and see the needs in our homes and our local communities to realise how many opportunities await us."
He also said prior to Mass that the martyrs "remind us of the importance that faith, moral rectitude and commitment to the common good have played, and continue to play, in the cultural, economic and political life of this country."
Gay rights activists had been calling on Pope Francis to denounce their persecution in Uganda, however he did not mention this specifically in his homily.
A law banning homosexuality increased attacks against gay people in the country, and even though the law was recently repealed, persecution continues and it is possible the law may be reintroduced.
Pope Francis said that "as members of God's family, we are to assist one another, to protect one another", and urged the Ugandan Church to "assist young couples to prepare for marriage, to encourage couples to live the marital bond in love and fidelity, and to assist parents in their duty as the first teachers of the faith for their children."
He is scheduled to arrive in Central African Republic on Sunday.
Security will be tight after recent killings by both Christian and Muslim militia.