He that begun would finish. - Matthew Henry
Thou shalt not be afraid of them: but shalt well remember what the LORD thy God did unto Pharaoh, and unto all Egypt; The great temptations which thine eyes saw, and the signs, and the wonders, and the mighty hand, and the stretched out arm, whereby the LORD thy God brought thee out: so shall the LORD thy God do unto all the people of whom thou art afraid. - Deut. 7:18-19
These verses are part of a passage in which God is encouraging the people as they begin claiming and conquering the Promised Land. He wanted them to focus, not on their fear, but on what He had already done for them. He reminds them of the work He did in Egypt on their behalf and encourages them by telling them that He would continue to do the same in the future.
I've often heard it said that it's a good idea to record the things that the LORD's done for us. That record serves as a reminder to us of His faithfulness. Being able to look back at His work in our lives in black and white increases our faith and spurs us on to keep walking in it by reminding us that He is always faithful, hedging us before and behind (Psalm 139:5). What He starts, He finishes. We can count on it.
Remember his marvelous works that he hath done; his wonders, and the judgments of his mouth . . .
- Psalm 105:5