John Gill Commentary Psalms 94:18

"When I said, My foot slippeth; Thy lovingkindness, O Jehovah, held me up." — Psalms 94:18 (ASV)

When I said, my foot slips
There is no ground for me to stand upon; all is over with me; there is no hope nor help for me; I am just falling into ruin and destruction: such will be the desperate case of the church, at the time before referred to:

your mercy, O Lord, held me up ;
the extremity of his people is the Lord's opportunity; then is his set time to arise, and have mercy on them; then mercy steps in, lays a solid ground and foundation for hope, and holds up in its arms a sinking people, and revives a dying cause.