Elder Aimilianos of Simonopetra | Joyfulness of heart is the undiminished sense you have that God is with you.

Joyfulness of heart is the undiminished sense you have that God is with you. It is a feeling of complete assurance, a total entrusting of your self to God. It's that magnificent sense of openness, such as we experience in states of rest and relaxation, a feeling of renewal that a person has when he is certain that, where he is, God is too. As such it is the feeling of a man who realizes that he is no longer alone, that there is not even a single moment when he is on his own. Joyfulness of heart, then, is the feeling, the deeper experience, that a person has when he knows that God is with him at every moment.

Archimandrite Aimilianos of Simonopetra