en_tn/1jn/03/19.md

we are of the truth

This could be translated as "we are living according the the way Jesus taught us."

assure our heart

In 3:19-22 the word "heart" refers to a person's conscience or the part of a person's thinking through which God makes him aware that he is doing something sinful. This can be translated as "we will not feel guilty in the presence of God." (UDB) (See: rc://en/ta/man/translate/figs-metonymy)

God is greater than our heart

This could be translated as "we know God is a better judge than our heart."

and do the things that are pleasing in his sight.

This can be translated "we do what pleases him"