shall declare His righteousness = His righteousness shall be declared.
that shall be born. Referring to the new birth as declared by Christ to Nicodemus (Joh_3:3-7). Compare Eze_36:25-27.
That = For. Corresponding with "for", Psa_22:24 and Psa_22:28, not Psa_22:21.
He hath done this = It is finished. Compare Joh_19:30. Thus concluding the Psalm. Compare the beginning. Hebrew. 'asah, to accomplish or finish, as in 2Ch_4:11.