Many believe - and I believe - that I have been designated for this work by God. In spite of my old age, I do not want to give it up; I work out of love for God and I put all my hope in Him.

This quote is saying that the speaker is quite old, but despite this, they feel a strong belief that God has chosen them for a certain task and they are determined to carry it out out of devotion to God. They are placing all of their trust and faith in God and they are refusing to give up the work, despite their age.