Avraham chosen for his future, not past

עיין ברמב"ן שהקשה למה לא הקדים הכ' שהי' אברהם צדיק כמו בנח.

See the Ramban, who asked: why did the verse not first state that Avraham was a tzaddik, as it did with Noach?

The Sefas Emes opens with the Ramban's question — Scripture introduces Noach by praising him as a righteous man, yet gives no such introduction for Avraham before Hashem commands him to leave his land.

אבל אין זה קושיא כי עיקר שלימות אאע"ה הי' אח"כ שנשתנה להיות נימול ונעשה ברי' חדשה כדכ' ואעשך ע"ש במדרש.

But this is no difficulty, for the essential perfection of Avraham Avinu came afterward, when he was changed to become circumcised and became a new creation, as it is written, "and I will make you [into a great nation]" (Bereishis 12:2) — see the Midrash there.

The Sefas Emes answers that Avraham's true completeness did not lie in his earlier state but came later, through the bris milah, by which he became a wholly new being — as the Midrash reads "and I will make you" to mean Hashem would refashion him anew.

והלא צוהו לך לך כו' שהורה לו דרך חדש לגמרי.

For indeed He commanded him, "Lech lecha (Go forth)," etc., which showed him an entirely new path.

The very command of "Lech lecha" set Avraham upon a completely new road in avodas Hashem, one he had not walked before.

ואיך יתלה הכ' כחו של אאע"ה במעשיו הקודמים.

So how could the verse have attributed the strength of Avraham Avinu to his earlier deeds?

It would have been inappropriate for the Torah to root Avraham's greatness in his prior conduct, since his real distinction lay in the new path that still lay ahead of him.

אך זה אמת על ידי שהי' לו אהבה רבה אליו ית' זכה להמשיך דרך חדש לעולם.

However, this is true: through having a great ahavah (love) for Him, blessed be He, he merited to draw a new path into the world.

What did enable Avraham to open this new road was his immense love for Hashem; that love is what earned him the power to bring an entirely new way of serving Hashem into the world.

אבל עיקר בחירת השי"ת בו בזכות מה שעשה אח"כ וזכות בני ישראל היוצאים ממנו:

But the essential reason Hashem chose him was in the merit of what he would do afterward, and in the merit of Bnei Yisrael who would issue from him.

Ultimately, Hashem's choice of Avraham rested not on his past but on his future deeds and on the merit of the Jewish people destined to descend from him.

Summary: The Torah does not introduce Avraham as a tzaddik (as it does Noach) because Avraham's true greatness lay not in his earlier deeds but in what lay ahead — the bris milah that made him a new creation and the new path of avodah he would open. His great love for Hashem earned him this, and Hashem chose him in the merit of his future and of Bnei Yisrael who would descend from him.