If someone is uncertain about the sincerity of their intentions for a religious act, they can take several steps to purify and strengthen their intentions:

1. **Self-Reflection**: Engage in sincere self-reflection to assess the motivation behind the religious act. Consider whether the primary aim is to seek God’s pleasure and adhere to Islamic principles rather than seeking personal gain, recognition, or approval from others.

2. **Renew Intentions (Niyyah)**: Actively renew and clarify your intentions before performing the act. Remind yourself that the purpose of the act is to please God and fulfill religious obligations.

3. **Supplication (Dua)**: Make supplications to God, asking for guidance and sincerity in your intentions. Request God’s assistance in purifying your heart and making your actions purely for His sake.

4. **Avoid Show-Off (Riya)**: Be mindful of avoiding actions that may lead to showing off or seeking admiration from others. The Prophet Muhammad (peace be upon him) emphasized that actions should be done solely for the sake of God.

5. **Increase Awareness of God (Taqwa)**: Strengthen your consciousness of God through regular prayers, recitation of the Quran, and other acts of worship. This increased awareness can help align your intentions with the divine purpose.

6. **Seek Knowledge**: Learn more about the principles of sincerity in Islam from knowledgeable scholars or Islamic resources. Understanding the concept of Ikhlas (sincerity) can help clarify and strengthen your intentions.

7. **Consult with a Religious Advisor**: If uncertainty persists, discuss your concerns with a trusted religious advisor or scholar. They can offer personalized guidance and support.

By taking these steps, you can work towards purifying your intentions and ensuring that your religious acts are performed with genuine sincerity and devotion.