I was cast upon thee from the womb
Either by himself, trusting in God, hoping in him, and casting all the care of himself upon him; or by his parents, who knew the danger he was exposed to, and what schemes were laid to take away his life; and therefore did, in the use of all means they were directed to, commit him to the care and protection of God: the sense is, that the care of him was committed to God so early; and he took the care of him and gave full proof of it:
thou [art] my God from my mother's belly:
God was his covenant God from everlasting, as he loved his human nature, chose it to the grace of union, and gave it a covenant subsistence; but he showed himself to be his God in time, and that very early, calling him from the womb, and making mention of his name from his mother's belly, and preserving him from danger in his infancy; and it was his covenant interest in God, which, though mentioned last, was the foundation of all his providential care of him and goodness to him. Now all these early appearances of the power and providence of God, on the behalf of Christ as man, are spoken of in opposition to the scoffs and flouts of his enemies about his trust in God, and deliverance by him, and to encourage his faith and confidence in him; as well as are so many reasons and arguments with God yet to be with him, help and assist him, as follows.