The words "calling" and"election" share similar meanings and refer to God's choosing them to belong to him. Alternate translation: "make sure that God has really chosen you to belong to him" (See: [[rc://en/ta/man/translate/figs-doublet]])
Here the word "stumble" refers either to 1) committing sin. Alternate translation: "you will not practice sinful behavior" or 2) becoming unfaithful to Christ. Alternate translation: "you will not become unfaithful to Christ" (See: [[rc://en/ta/man/translate/figs-metaphor]])