(some fundamentals of releasing & elevating hidden & trapped sparks)
This stance helps filter out the souls lacking an authentic Yisrael spark.
Every convert who means to fully accept the yoke of Torah & all applicable mitzvot receives a new soul.
However, different levels of soul come according to the convert's level of intention.
For example, Ruth's conversion reached one of the highest levels, which merited her an extraordinarily special Yisrael soul.
At the same time, all sincere converts receive a special status with a direct lineage to Avraham Avinu & Sara Imeinu!
Furthermore, Hashem bequeaths converts with special protection, as commanded directly from the Torah and described in Tehillim 146:9 — Hashem guards & protects specifically the ger (even as He is, of course, the All-Powerful Shomer Yisrael).
We see that the great David HaMelech is the great-grandson of a convert (Ruth), which means Melech HaMashiach is also descended from a convert.
Rebbi Akiva was the son/grandson of converts & a direct descendent of the abominable General Sisera — yet Rebbi Akiva became one of Am Yisrael's greatest Sages.
So we see how wholly sincere conversion produces very special results.
The information in the above links, mostly gleaned from the great tzaddik Rav Levi Yitzchak Bender, explains why a truly kosher & sincere conversion is so powerful & so important for Am Yisrael.
After learning about the sparks & what conversion really does, both for the convert & for Am Yisrael, it's easy to deduce why invalid or questionable conversions are so damaging, not only practically, but also on a spiritual level.
Often, a potential convert contains a Jewish spark, which leads the soul toward conversion.
Hashem planted that spark within because, for whatever reason, this person needed the process of conversion to carry out his or her soul rectification.
For example, some converts left Judaism in their former gilgul (lifetime).
To repair that terrible heresy, Hashem causes them to be born as non-Jews in a new gilgul — non-Jews who actively need to pursue Judaism & proactively adopt the full yoke of Torah & mitzvot to atone for the rejection they performed in their previous gilgul.
And that's just one interesting & newer reason. There are other better-known, all-encompassing reasons, such deriving from the sparks of souls nursed by Sara Imeinu, or who wanted to accept the Torah when their nation did not, and more.
Regardless, it's vital that a conversion be carried out properly.
The rabbis involved must be Orthodox—but being Orthodox isn't enough.
And good intentions are not enough either.
In fact, good intentions often go hand-in-hand with insincere conversions carried out by otherwise Orthodox rabbis.
The rabbi much check his personal motivations and not slide down the slippery slope of "feel-good" impulses.
These rabbis must deal with conversion responsibly & take it very seriously.
It's not enough for them to THINK they're being responsible and taking it seriously. They must actual BE responsible and take it very seriously with all the halachah that entails.
If they don't, they risk contributing to an insincere conversion (as has happened numerous times & continues to happen).
The following short video contains a directive from the Lubavitcher Rebbe about how to conduct a kosher conversion & how to determine the sincerity of the potential convert.
It contains wonderful direction & I wish every Orthodox rabbi would use this method when dealing with potential converts: