Praise be to the God and Father of our Lord Jesus Christ, who has blessed us in the heavenly realms with every spiritual blessing in Christ. For he chose us in him before the creation of the world to be holy and blameless in his sight. In love he predestined us for adoption to sonship through Jesus Christ, in accordance with his pleasure and will— to the praise of his glorious grace, which he has freely given us in the One he loves. In him we have redemption through his blood, the forgiveness of sins, in accordance with the riches of God's grace that he lavished on us. With all wisdom and understanding, he made known to us the mystery of his will according to his good pleasure, which he purposed in Christ.
Two sets are disjoint if they have no elements in common. In this case, we write A ÇB = Æ. Disjoint sets of objects are easy to create; consider, for example, the sets A = {1, 3, 5} and B = {2, 4}. These sets have no numbers in common.
Okay, so what about people? Are there disjoint sets of people? Many would like to claim that we are separated by race or creed or nationality. But through Christ we know that in God's eyes these are not true distinctions (see Gal 3:28). God does separate people into a pair of disjoint sets (which is actually a partition): people who are redeemed and people who are not redeemed. Those who are redeemed are adopted heirs of salvation through faith in Jesus. However, people are not given the insight to determine who is in each of these sets. It is the commission of redeemed people to spread the good news of salvation to all people and let God sort out who will be in each set.