All believers are purified & infallible

[Husayn]

Let's do a wacky interpretation:

يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا قُمْتُمْ إِلَى الصَّلَاةِ فَاغْسِلُوا وُجُوهَكُمْ وَأَيْدِيَكُمْ إِلَى الْمَرَافِقِ وَامْسَحُوا بِرُءُوسِكُمْ وَأَرْجُلَكُمْ إِلَى الْكَعْبَيْنِ ۚ وَإِن كُنتُمْ جُنُبًا فَاطَّهَّرُوا ۚ وَإِن كُنتُم مَّرْضَىٰ أَوْ عَلَىٰ سَفَرٍ أَوْ جَاءَ أَحَدٌ مِّنكُم مِّنَ الْغَائِطِ أَوْ لَامَسْتُمُ النِّسَاءَ فَلَمْ تَجِدُوا مَاءً فَتَيَمَّمُوا صَعِيدًا طَيِّبًا فَامْسَحُوا بِوُجُوهِكُمْ وَأَيْدِيكُم مِّنْهُ ۚ مَا يُرِيدُ اللَّـهُ لِيَجْعَلَ عَلَيْكُم مِّنْ حَرَجٍ وَلَـٰكِن يُرِيدُ لِيُطَهِّرَكُمْ وَلِيُتِمَّ نِعْمَتَهُ عَلَيْكُمْ لَعَلَّكُمْ تَشْكُرُونَ

O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles. And if you are in a state of janabah, then purify yourselves. But if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and do not find water, then seek clean earth and wipe over your faces and hands with it. Allah does not intend to make difficulty for you, but He intends to purify you and complete His favor upon you that you may be grateful.

Qur'an 5:6

Here, Allah (swt) says "liyutahhirakum", the same thing he says in Ayat at-Tathir (wa yutahhirakum).

Sunnis say (in regards to 33:33) - Allah (swt) means to purify Ahlul Bait by encouraging them to avoid sins, and "yutahhirakum" is a wish of Allah (swt).

Shiis say (in regards to 33:33) - no, Allah (swt) means that He has completely purified certain individuals, and that "yutahirrakum" is a definitive statement declaring their eternal purity (these certain individuals).

So fine, applying this logic to 5:6, all the believers are now eternally tahir and ma'soom.

[Rationalist]

On top of that in the theology of the 12ers, you are still a momin even after you sin as long you are a 12er. However, a non-12er can never be a momin. This is contrary to the ahadith narrated in al Kafi.

[Hani]

here's what's similar:
http://forum.twelvershia.net/quran-tafseer/verse-of-purification-and-verse-of-guidance-for-the-sahabah/