4 occurrences across the corpus

King James Version

Genesis 43:3

And Judah spake unto him, saying, The man did solemnly protest unto us, saying, Ye shall not see my face, except your brother be with you.

1 Samuel 8:9

Now therefore hearken unto their voice: howbeit yet protest solemnly unto them, and shew them the manner of the king that shall reign over them.

KJV Apocrypha (Catholic Deuterocanon)

2 Maccabees 14:4

Came to king Demetrius in the hundred and one and fiftieth year, presenting unto him a crown of gold, and a palm, and also of the boughs which were used solemnly in the temple: and so that day he held his peace.

Apostolic Fathers

Shepherd of Hermas: Similitudes 9:25

And they who believed from the eighth mountain, where were the many fountains, and where all the creatures of God drank of the fountains, were the following: apostles, and teachers, who preached to the whole world, and who taught solemnly and purely the word of the Lord, and did not at all fall into evil desires, but walked always in righteousness and truth, according as they had received the Holy Spirit. Such persons, therefore, shall enter in with the angels.”