Steadfastness does not mean never making a mistake. It means refusing to settle into a knowingly wrong pattern and returning to the right course after a lapse. Keeping the same moral principles in worship, transactions, promises, and speech is its practical expression.
Example 1: Refuse to cheat in an exam even when no supervisor is watching.
Example 2: If you have wronged someone, return their right and change your future conduct instead of merely offering an excuse.
The ending تُ in آمَنْتُ means “I,” whereas اسْتَقِمْ addresses “you.” The statement of belief is followed by a command to live accordingly.