No, not all Wehrmacht personnel were members of the Nazi Party. According to Richard Evans, one of the leading experts on Nazi Germany, as of 1941, about 29% of the senior officer corps were Nazi Party members, while rates of membership were slightly higher among junior officers. These younger officers (as well as younger conscripts) had been educated and trained in the Nazi system for most or all of their military careers, and were more thoroughly indoctrinated with Nazi ideology than the more senior officers, most of whom had been trained and developed during the Imperial period or the Weimar Republic. I'm not aware of any comprehensive statistical data on the percentage of enlisted men who were members of the Nazi Party, but we're talking about a group of about 18,000,000 people here, so naturally there were differences in political beliefs and motivations among them. However, Wehrmacht personnel at all levels of the military hierarchy were subjected to Nazi propaganda during the period leading up to the Second World War and during the war itself, so the ideological foundations of the Nazi worldview were clearly conveyed to them.

There was certainly some resistance to Hitler and Nazism, particularly among the older guard of Prussian officers during the Nazis' initial rise to power. However, Hitler mostly assuaged those concerns by purging the SA, a Nazi paramilitary force that the traditional military leadership despised and feared due to their reputation for ill discipline and street violence, as well as by abrogating the terms of the Treaty of Versailles and rebuilding the German military with no regard for the restrictions that were imposed by the Treaty, which the traditional officer corps resented just as strongly as the Nazis did. There was also internal resistance during the war, most notably in the case of the attempted assassination of Hitler on 20 July 1944. This plot was led by a German colonel, Claus von Stauffenberg. Obviously the assassination failed, and some 5,000 Wehrmacht personnel were executed for their real or alleged participation in the plot (famously including Field Marshal Erwin Rommel, who was forced to commit suicide even though the extent of his involvement is uncertain and remains controversial). A few German military personnel are known to have helped rescue Jews during the Holocaust. Among the best-known is Wilm Hosenfeld, a German captain who helped Jews, including the pianist Władysław Szpilman, survive in Warsaw during the final months of the war. His actions were depicted in the film The Pianist, based on Szpilman's memoir, and he was later named Righteous Among the Nations by Yad Vashem for his efforts.

However, despite these instances of resistance and aid, the Wehrmacht was committed numerous war crimes and was actively involved in the perpetration of the Holocaust. The idea of an apolitical Wehrmacht, which conducted an honorable war despite Hitler's interference and was wholly unaware of the Holocaust, is known as the "myth of the clean Wehrmacht". This myth was actively fostered by surviving German military personnel after the war (most notably Franz Halder, who worked for the US Army Historical Division) and gained widespread currency in the West, particularly as the West's interests shifted from prosecuting German war crimes to confronting the threat of the Soviet Union. It took decades for historians to fully correct the record on this issue and demonstrate the full extent of the Wehrmacht's crimes. This process culminated in the "Wehrmacht Exhibition" (Wehrmachtsausstellung) in Hamburg in 1995, which displayed displayed over a thousand photos of Wehrmacht war crimes, including those committed by ordinary rank-and-file soldiers. The modern historical consensus has fully rejected the "clean Wehrmacht" myth, even though it unfortunately still has currency in popular culture.

The historical evidence against the "clean Wehrmacht" myth begins with the so-called "criminal orders" which were issued before and during the invasion of the Soviet Union. Hitler explicitly framed the war against the Soviet Union as a war of ideologies (Weltanschauungskrieg) and a war of racial extermination (Vernichtungskrieg), and this was reflected in the orders that were issued to the troops on the Eastern Front. The first, the Barbarossa Decree, authorized the summary execution of Soviet civilians, while the Commissar Order commanded German troops to immediately execute captured Soviet political commissars. Some apologist historians attempted to claim that these orders weren't carried out, or at least not widely carried out, but work by German historians like Felix Römer has demonstrated that that claim is nonsense, since over 80% of German divisions reported the execution of captured political commissars. These orders were obviously in direct contravention of the existing international law regarding the treatment of prisoners of war and civilians, and contrary to any sort of honorable military tradition, but they were carried out anyway due to both the effectiveness of Nazi propaganda in portraying Slavs and Jews as subhumans (Untermenschen) who had to be eliminated so that Germany could prosper, as well as the simple tendency within a military hierarchy to follow superior orders (summarized by the German phrase "Befehl ist Befehl", meaning "an order is an order").

German troops were subjected to intensive Nazi racial propaganda both before and during the war, and we now know that, despite the postwar mythmaking, the Wehrmacht was intimately involved in the mass murder of people who were deemed racially inferior by Nazi ideology and were thus active participants in Hitler's planned racial-ideological war of extermination. The most prominent example is the Wehrmacht's treatment of Soviet POWs. While the Germans generally followed the requirements of the Geneva Convention of 1929 in their treatment of other Allied POWs, Soviet POWs were marked for extermination as part of the German plan to depopulate Eastern Europe and repopulate it with Germans (Generalplan Ost). Of the 5.7 million Soviet troops the Germans captured during the war, 3.3 million, or 58%, died in captivity. Most of these deaths were due to deliberate starvation, as well as exposure and diseases that spread rapidly in the primitive, overcrowded, unsanitary POW camps. The Germans took about 3.3 million Soviet prisoners during Operation Barbarossa in 1941, and it's estimated that about 2 million of them had died by February 1942. By the fall of 1941, the death rate for Soviet POWs in German captivity was 1% per day. At that point, the Soviet prisoners were dying at higher rates than Jewish victims of Operation Reinhard (the most concentrated period of killing during the Holocaust). After the failure of Operation Barbarossa, conditions improved somewhat as the Germans decided to use the prisoners as forced laborers to support the German war effort, rather than simply killing them outright, but another 1.3 million prisoners still died before the end of the war. The camps in which this mass death took place were run by Wehrmacht officers and guarded by German military personnel (generally reservists, but sometimes active duty troops, especially in the transit camps near the front), so the Wehrmacht was directly responsible for the deaths of the prisoners.

The Wehrmacht was also actively involved in killing Jews, and the idea that they were unaware of the Holocaust is completely untenable. Jewish-Soviet POWs were separated from other prisoners in the German POW camps and were transported to concentration camps where they were summarily executed. We also know that in some cases, Jewish civilians passed through German POW camps on their way to concentration camps, and that the Wehrmacht operated its own "concentration camps" for civilians (Jewish and non-Jewish) in the occupied Soviet Union (though it should be noted that despite their name, these camps were not part of the main concentration system that was run by the SS). Although the mass shootings of Jews on the Eastern Front (the so-called "Holocaust by bullets") was primarily carried out by the SS mobile killing units (Einsatzgruppen) with the assistance of local collaborators, Wehrmacht personnel were also involved in some cases, both as triggermen as well as providing logistical support for the killing operations. Wehrmacht troops also carried out executions of captured "partisans", many of whom were not partisans, but ordinary civilians who were killed as part of a collective reprisal for nearby partisan activity. The Wehrmacht also participated in other actions that aren't as well known, such as the execution of psychiatric patients in institutions in the occupied Soviet Union. While it's true that not all German soldiers participated in these crimes, it's equally true that there's no way anyone could credibly claim not to have known what was happening to Jews and Soviet POWs.